Back to EveryPatent.com
| United States Patent |
5,751,185
|
|
Yamamoto
, et al.
|
May 12, 1998
|
Low pass filter circuit utilizing transistors as inductive elements
Abstract
The present invention relates to a filter circuit which is suitable for use
in an integrated circuit formed of transistors or field effect
transistors, and controls the frequency band of the amplifying circuit
used for the signal receiver, for example, within an optical
communications system. The filter circuit includes a first transistor (T1)
having a base or gate grounded with high-frequency component via a first
resistor (R1); and a second resistor (R2) and a capacitor (C1) each having
one end connected to the emitter or source of the first transistor (T1)
and the other terminal grounded with high-frequency component. The filter
circuit acts as a circuit in which the first transistor (T1) inputs the
current (Iin) at a juncture at which the second resistor (R2) and the
capacitor (C1) are connected therewith and outputs its collector or drain
current (Iout).
| Inventors:
|
Yamamoto; Takuji (Kawasaki, JP);
Kuwata; Naoki (Kawasaki, JP)
|
| Assignee:
|
Fujitsu Limited (Kanagawa, JP)
|
| Appl. No.:
|
827780 |
| Filed:
|
April 11, 1997 |
Foreign Application Priority Data
| Jul 27, 1993[JP] | 5-184256 |
| Feb 09, 1994[JP] | 6-014733 |
| Current U.S. Class: |
327/558; 327/552 |
| Intern'l Class: |
H03H 001/00 |
| Field of Search: |
327/336,344,552,553,555,558
|
References Cited
U.S. Patent Documents
| 3517223 | Jun., 1970 | Gaunt, Jr. | 327/552.
|
| 3577016 | May., 1971 | Weldmann | 327/558.
|
| 4087757 | May., 1978 | Cunningham | 327/553.
|
| 4363004 | Dec., 1982 | Englund, Jr. et al. | 327/553.
|
| 4424462 | Jan., 1984 | Gay | 327/558.
|
| 4877979 | Oct., 1989 | Sempel | 327/558.
|
| 5099155 | Mar., 1992 | Mohwinkel | 327/558.
|
| 5293087 | Mar., 1994 | Hamano et al. | 327/558.
|
| Foreign Patent Documents |
| 0 125 426 | Nov., 1984 | EP.
| |
| 49-36839 | Apr., 1974 | JP.
| |
| 49-148838 | Dec., 1974 | JP.
| |
| 51-40737 | Apr., 1976 | JP.
| |
| 54-135137 | Sep., 1979 | JP.
| |
| 56-166616 | Dec., 1981 | JP.
| |
| 62-160809 | Jul., 1987 | JP.
| |
| 2-44425 | Mar., 1990 | JP.
| |
| 2-202212 | Aug., 1990 | JP.
| |
| 4-23607 | Jan., 1992 | JP.
| |
| 4-150513 | May., 1992 | JP.
| |
| 2 077 559 | Dec., 1981 | GB.
| |
Other References
J. Choma et al. "Actively Peaked Broadbanded Monolithic Amplifier" IEEE
Proc. vol. 127 Pt. G No. 2 Apr. 1980.
J. Choma et al. "A Broadbanded Common Collector Common-Base Differential
Quartet" IEEE Journal of Solid State Circuits vol. ed-16, No. 2, Apr.
1981.
J.Choma, Jun "Output Inductance of an Emitter Follower", Electronic
Circuits and Systems, vol. 3 No. 4 Jul. 1979.
Simpson, "Two Active Low-Pass Instrumentation Filters", Aug. 1971, p. 45.
|
Primary Examiner: Cunningham; Terry
Attorney, Agent or Firm: Helfgott & Karas, P.C.
Parent Case Text
This is a continuation of application Ser. No. 08/689,386, filed Aug. 12,
1996, now abandoned, which is a continuation of application Ser. No.
08/387,876, filed Feb. 22, 1995, now abandoned, which is a continuation of
PCT/JP94/01234 filed Jul. 26, 1994.
Claims
We claim:
1. A low pass filter circuit having a secondary cut-off frequency
characteristic comprising:
a first transistor T1 having a base or gate connected to a voltage point
through a first resistor R1, said voltage point is stable to
high-frequency components;
a second resistor R2 and a capacitor C1 each having one end connected to an
emitter or source of said first transistor T1 and the other end connected
to said voltage point which is stable to high-frequency components; and
wherein said first resistor R1 and said first transistor T1 function as an
inductive element of said circuit having an inductance value Leq1 defined
by: Leq.sub.1 =R1/2.pi.fT, where fT is the transition frequency of T1.
2. A low pass circuit having a secondary cut-off frequency characteristic
comprising:
a first transistor T7 having a base or gate connected to a first resistor
R10, a collector or drain connected to a first voltage point which is
stable to high-frequency components, and an emitter or source connected to
a constant current source CCS3;
a second resistor R11 and a capacitor C5 each having one terminal connected
to the emitter or source of said first transistor T7 and the other
terminal connected to a second voltage point which is stable to
high-frequency components;
wherein said first resistor R10 and said first transistor T7 function as an
inductive element of said circuit having an inductance value Leq2 defined
by: Leq2=R10/2.pi.fT, where fT is the transition frequency of T7.
Description
TECHNICAL FIELD
The present invention relates to a filter circuit which is suitable for
conversion to an integrated circuit formed of transistors or field-effect
transistors and controls the frequency band of the amplifying circuit used
for the signal receiver, for example, within an optical communications
system.
BACKGROUND ART
In the receiving unit arranged in an optical communications system, plural
filter circuits are used in accordance with purposes. That is, the
equalizing and amplifying circuit needs a low-pass filter circuit to have
the optimum frequency band and optimum cut-off characteristic determined
in the waveform interference/noise band tradeoffs. The timing-clock
amplifying circuit needs a low-pass filter, a high-pass filter, or a
band-pass filter of which each suppresses the jitter due to circuit noises
and prevents erroneous operation due to harmonic components.
Recently, the optical communications systems which operate at a high
transmission speed of 2.4 Gb/s are already in an actual use. The
development of the system operating as speed as 5 Gb/s to 10 Gb/s is being
spurred. With this system development, the filter circuit must operate at
operable frequencies of several GHz to 10 GHz to be used for the
amplifying circuit.
Moreover, the filter circuit which can vary its cut-off frequency and
center frequency is needed in order to share a filter circuit used in a
system which operates at a different transmission speed and to suppress
the characteristic deviation due to variations between the circuit forming
components and characteristic variation due to characteristic drifts in
circuit elements.
In the optical communications systems, the system forming circuits have
evolved from discrete-parts assembled circuits into integrated circuits.
Moreover, the integrating technology with which analog circuits and
digital circuits are formed on a semiconductor chip has been in practical
use. Hence it is desirable to match the filter circuit to the
above-mentioned technology. The active filter circuit can respond to such
a demand.
An active filter circuit formed of an operational amplifier and a CR
circuit has been first developed. The reason is that utilizing the high
gain, very high input impedance, and low output impedance of the
operational amplifier allows the simple circuit design.
However, the operational amplifier cannot be used for the above-mentioned
high-speed system because its effective frequency range is narrow. Hence,
the art which can construct an active filter circuit formed of wide-range
operable active elements has been more concerned importantly.
In order to deal with plural transmission speeds with the same integrated
circuit and to provide general versatility, a filter circuit that can vary
its frequency over a wide range is required.
FIG. 48 is a diagram showing the constructional example of the optical
signal receiving unit arranged in an optical communications system.
Referring to FIG. 48, numeral 1 represents a light receiving element, 2
represents an equalizing and amplifying circuit, 3 represents a slicing
and amplifying circuit, 4 represents a timing extracting circuit, 5
represents a filter, 6 represents a limiting and amplifying circuit, and 7
represents a decision circuit.
The light receiving element 1 converts an optical signal into an electrical
signal. The equalizing and amplifying circuit 2 amplifies linearly an
electrical signal converted by the light receiving element 1 to a
predetermined amplitude. The slicing and amplifying circuit 3 slices the
signal amplified by the equalizing and amplifying circuit 2.
The timing extracting circuit 4 extracts a frequency timing of the signal
amplified by the equalizing and amplifying circuit 2 to output clocks
pulses. The filter 5 reduces noises included in the clock. The limiting
and amplifying circuit 6 limit-amplifies the clock passed through the
filter 5 to a predetermined amplitude.
The decision circuit 7 discriminates the data signal amplified by the
slicing and amplifying circuit 3 in response to the clock signal amplified
by the limiting and amplifying circuit 6 to output data.
The optical signal receiving unit with the above-mentioned structure can
amplify the signal weakened due to the long distance transmission in an
optical communications system.
In the equalizing and amplifying circuit 2, the S/N ratio is degraded
because of the widened noise range in an excessively widened frequency
range. On the contrary, the receiving sensitivity is degraded because of
an increased waveform interference when the frequency range is excessively
narrowed.
Since the equalizing and amplifying circuit 2 has the optimum frequency
range according to the applicable transmission speed in the equilibrium of
noise range and waveform interference, it is necessary to build an active
low-pass filter circuit with good performance in the equalizing and
amplifying circuit 2 in an integrated circuit form.
In order to suppress jitter due to internal noises therein and to prevent
an erroneous circuit operation due to harmonic components produced in its
limiting and amplification, the limiting and amplifying circuit 6 needs
its tuning characteristic. In order to realize the tuning characteristic
in the integrated circuit, the active filter circuit such as a low-pass
filter circuit, a high-pass filter circuit, or a band-pass filter is
needed.
In any cases, it is needed that the frequency-band variable filter circuit
can vary its characteristic to deal with range variations or unintentional
peaking due to manufactual variations of an integrated circuit including
the above-mentioned amplifying circuit.
A prior-art active filter circuit and frequency-range variable filter
circuit will be described below.
First, a prior-art active filter circuit will be described below.
FIG. 49 is a diagram showing a prior art active low-pass filter circuit.
Referring to FIG. 49, symbols T17a and T17b represent input transistors;
T18a and T18b represents resistors, or output transistors, each of which
decides the cut-off frequency; T19a and T19b represent capacitors each of
which decides the cut-off frequency; R18a and R18b represent resistors
each of which converts an input voltage into a current; R19a and R19b
represent resistors each of which converts a current into a voltage or
output voltage.
The circuit, shown in FIG. 49, is a RC filter circuit with a first-order
cut-off characteristic which is formed of a resistor counting each of the
transistors T18a and T18b and a capacitance counting each of the
transistors T19a and T19b.
If the current amplification constant .beta.o of each of the transistors
T18a and T18b is sufficiently large, the resistors counting the
transistors T18a and T18b correspond to the emitter resistor r.sub.e,
respectively. Because the juncture between the emitters of the transistors
T19a and T19b is regarded as an imaginary ground, the capacitance counting
the transistors T19a and T19b is the sum of the collector junction
capacitance CJC, the emitter junction capacitance CJE, and the emitter
diffusion capacitor Cd. Hence the cut-off frequency fc is expressed by the
following formula:
fc=1/›2.pi.r.sub.e (Cd+CJE+CJC)!
The filter circuit, shown in FIG. 49, can adjust its cut-off frequency by
controlling the bias current of each of the transistors T19a and T19b
because the emitter diffusion capacitance Cd is proportional to the
emitter current (in a normal bias condition of Cd>>CJE, CJC is
established).
This circuit can be used as a balanced circuit in which an input voltage is
applied between the base of the transistor T17a and the base of the
transistor T17b to produce the output between the collector of the
transistor T18a and the collector of the transistor T18b, an unbalanced
circuit in which an input voltage is input between the base of the
transistor T17a or T17b and the ground to produce an output between the
collector of the transistor T18a or T18b and the ground, or a circuit
which has a balanced input and an unbalanced output, and vice versa.
However, since the circuit, shown in FIG. 49, is a primary filter circuit
(with a first-order cut-off characteristic), the secondary filter circuit
(with a second-order cut-off characteristic) may be realized by cascading
the same circuits (shown in FIG. 49) or by using a LCR filter circuit with
secondary characteristic) including a coil (L) and a capacitor (C).
In the former case, there is a disadvantage in that cascading the primary
active filter circuits decreases the cutoff frequency. The practical
active filter circuit needs the voltage shift circuit added therein, shown
in FIG. 49, thus causing the large circuit scale.
Turning to the latter case, in order to realize the LCR filter circuit with
a secondary characteristic formed of coils and capacitors, there is a
disadvantage in that a coil cannot be suited to the integrated circuit and
the complicated coil equivalent circuit makes it difficult to control the
characteristic.
Next, a prior art frequency band variable filter circuit will be described
below.
FIGS. 50 to 52 are diagrams each showing the structural example of a prior
art band variable filter circuit.
In the circuit, shown in FIG. 50, the casaded transistors 401a and 401b
have emitters connected respectively to the bases of a differential pair
of the transistors 501a and 501b. Thus, the frequency band control circuit
is exampled that cuts frequencies by means of the emitter resistors
r.sub.e of the transistors 401a and 401b and the emitter diffusion
capacitance Cd (.varies.IE) each of the differential pair of the
transistors 501a and 501b.
The frequency range of the filter circuit can be controlled by controlling
the emitter current IE of each of the differential pair of the transistors
501a and 501b by means of the variable current source 701b.
The frequency range f of the filter circuit, shown with a 3dB down region,
is decided by the frequency range fORG of a filter circuit with no
frequency band control circuit and the frequency range fCNTL of a filter
circuit with a frequency band control circuit.
In this case, the frequency range fCNTL of the filter circuit with a
frequency band control circuit is expressed by the formula:
fCNTL=1/2.pi.r.sub.e ›Cd(Ie)+CJE+CJC!, where CJE is the base-emitter
junction capacitance of each of the differential pair of the transistors
501a and 501b, CJC is the base-collector capacitance of each of the
differential pair of the transistors 501a and 501b, and r.sub.e is an
emitter resistor of each of the transistors 401a and 401b.
The maximum frequency range fCNTL(MAX) and the minimum frequency range
fCNTL(MIN) of the frequency band control circuit are respectively
expressed as follows:
fCNTL(MAX)=1/2.pi.r.sub.e (CJE+CJC)
fCNTL(MIN)=1/2r.sub.e Cd(IE(MAX)
As apparent from the above-formulas, the frequency band control circuit is
formed so as to satisfy the relationship of fCNTL(MIN)<fORG<fCNTL(MAX),
the frequency range f of the entire circuit can be controlled within
fCNTL(MIN)<f<fORG.
FIG. 51 is a graph showing an example of the result of the frequency band
variable characteristics, simulated by means of the SPICE. As shown in
FIG. 50, the frequency band can be narrowed as the emitter current IE
increases.
The structural example of the filter circuit, shown in FIG. 52, utilizes
that the transition frequency fT (a frequency at which the current
amplification factor .beta. is 1 in the case of the emitter ground) of a
transistor depends on the collector current thereof. Thus, two
differential pairs are connected in parallel. The current varying circuit
controls the current branching through the two differential pairs at a
distribution ratio so that the frequency band is controlled.
In order to spread the variable frequency band width of the filter shown in
FIG. 50, it is needed to narrow more the minimum frequency range by
increasing the emitter current IE.
However, increasing the emitter current IE requires enlarging the sizes of
the transistors 501a and 501b in the control circuit. As a result, the
increased parasitic capacitance (CJE, CJC) of each of the control circuit
transistors 501a and 501b may degrade the maximum frequency band
fCNTL(MAX).
Since the frequency band f of the entire circuit becomes
fCNTL(MIN)<f<fCNTL(MAX) (<fORG), the maximum frequency range is narrowed,
compared with no control circuit. Hence there is a disadvantage in that it
is difficult to control the frequency variable width over a wide range.
Furthermore, there is a disadvantage in that the filter circuit shown in
FIG. 52 cannot stably set the frequency range because it is difficult to
control the current distribution ratio to adjust the frequency range.
The present invention is made to overcome the above mentioned problems. An
object of the present invention is to provide a filter circuit suitable
for integrated circuits which has a secondary cut-off characteristic
showing a wide, stable, variable frequency range without deteriorating the
maximum frequency range of the entire circuit.
DISCLOSURE OF INVENTION
According to the present invention, the filter circuit is characterized by
a first transistor having a base or gate grounded with a high-frequency
component via a first resistor; and a second resistor and a capacitor each
having one end connected to an emitter or source of the first transistor
and the other terminal grounded with a high-frequency component; whereby a
current is input at a juncture at which the second resistor, the
capacitor, and the first transistor are connected together and a collector
or drain current is output from the first transistor.
According to the present invention, the filter circuit is characterized by
a first transistor having a base or gate connected to a first resistor, a
collector or drain grounded with a high frequency component via an
impedance of zero ohms or more, and an emitter or source connected to an
equivalent current source; and a second resistor and a capacitor each
having one terminal connected to the emitter or source of the first
transistor and the other terminal grounded with a high frequency; whereby
a voltage is input to the other terminal of the first resistor and the
emitter or source voltage of the first transistor is output.
Furthermore, according to the present invention, the filter circuit is
characterized by a first transistor having a base or gate grounded with a
high frequency component via a first resistor, and a collector or drain
grounded with a high frequency component via an impedance of zero ohms or
more; and a second resistor having one terminal connected to the emitter
or source of the first transistor and the other terminal grounded with a
high frequency component; and a capacitor connected to the emitter or
source of the first transistor; whereby a current is input at a juncture
at which the second resistor and the emitter or source of the first
transistor are connected together and a current is output from the other
terminal of the capacitor.
According to the present invention, the filter circuit is characterized by
a first transistor having a base or gate grounded with a high frequency
component via a first resistor, a collector or drain grounded with a
high-frequency component via an impedance of zero ohms or more, and an
emitter or source connected to an equivalent current source; a second
resistor having one terminal connected to the emitter or source of the
first transistor and the other terminal grounded with a high-frequency
component; and a capacitor having one terminal connected to the emitter of
source of the first transistor; whereby a voltage is input to the other
terminal of the capacitor and the emitter or source voltage of the first
transistor is output.
Furthermore, according to the present invention, the filter circuit is
characterized by a first transistor having a base or gate grounded with a
high-frequency component via a first resistor, a collector or drain
grounded with a high-frequency component via an impedance of zero ohms or
more; a capacitor having one terminal connected to the emitter or source
of the first transistor and the other terminal grounded with a
high-frequency component; and a second resistor having one terminal
connected to the emitter or source of the first transistor; whereby a
current is input at a juncture at which the capacitor and the emitter or
source of the first transistor are connected together and a current is
output at the other terminal of the second resistor.
According to the present invention, the filter circuit is characterized by
a first transistor having a base or gate grounded with a high-frequency
component via a first resistor, a collector or drain grounded with
high-frequency component via an impedance of zero ohms or more, and an
emitter or source connected to an equivalent current circuit; a capacitor
having one terminal connected to the emitter or source of the first
transistor and the other terminal grounded with a high-frequency
component; and a second resistor having one terminal connected to the
emitter or source of the first transistor; whereby a voltage is input to
the other terminal of the second resistor and the emitter or source
voltage of the first transistor is output.
Moreover, according to the present invention, the filter circuit is
characterized by a first transistor having a base or gate grounded with a
high-frequency component via a first resistor and a collector or drain
grounded with a high-frequency via an impedance of zero ohms or more; and
a capacitor having one terminal connected to an emitter or source of the
first transistor and the other terminal grounded with a high-frequency
component; whereby a current is input at a juncture at which the capacitor
and the emitter or source of the first transistor are connected together
and the emitter or source voltage of the first transistor is output.
According to the present invention, the filter circuit is characterized by
a first transistor having a base or gate grounded with a high-frequency
component via a first resistor and an emitter or source connected to an
equivalent current source; and a capacitor having one terminal connected
to the emitter or source of the first transistor; whereby a voltage is
input to the other terminal of said capacitor and the collector or drain
current from the first transistor is output.
According to the present invention, the filter circuit may further include
a second transistor which has a collector or drain, and a base or gate
grounded with a high-frequency component via an impedance of zero ohms or
more; and an emitter or source connected to an equivalent variable current
source and connected to the base or gate of the first transistor.
The filter circuit according to the present invention may further include a
second transistor which has a collector or drain grounded with a
high-frequency component via an impedance of zero ohms or more and an
emitter or source connected to an equivalent variable current source and
connected to the base or gate the first transistor; whereby a voltage is
input to the base or gate of the second transistor.
Furthermore, according to the present invention, the filter circuit is
characterized by a signal path switching unit for switching input signal
transmission paths by varying the control voltage to be applied; frequency
band control units connected to the signal transmission path, each having
a different range of bandwidth; and a cascade transistor having an emitter
or source connected to the signal path switching unit via the frequency
band control units.
According to the present invention, the signal path switching unit includes
a common-emitter or common-source ground transistor having an emitter or
source grounded via a resistor of zero ohms and more, and transistors each
connected to the collector or drain of the common-emitter or common-source
ground transistor; and the frequency band control units includes a
frequency band control circuit connected respectively to the collectors or
drains of the transistors, and resistors with different impedances
connected respectively to the collectors or drains of the transistor; and
the cascode transistor has an emitter or source connected to the
collectors or drains of the transistors via the resistors; whereby a
signal voltage is applied to the base or gate of the common-emitter or
common-source ground transistor and control voltages are respectively
applied to the bases or gates of the transistors.
Furthermore, the present invention is characterized in that the filter
circuit may further include a differential pair configuration which
consists of common-emitter or common-source ground transistors each having
an emitter or source connected to a constant current source via the
resistors and at the juncture therebetween; transistors; resistors; the
frequency band control circuits; and the cascaded transistors.
The present invention is characterized in that the signal path switching
unit is formed of the common-emitter or common-source transistor grounded
via the resistor of zero ohms or more, whereby a signal voltage plus a
control voltage are input to the bases or gates of the common-emitter or
common-source transistors.
Furthermore, the present invention is characterized in that the filter
circuit may further include a differential pair configuration which
consists of common-emitter or common-source transistors each having an
emitter or source connected to a current source via resistors; the
common-emitter or common-source transistor; resistors; the frequency band
control circuits; and transistors.
According to the present invention, the signal switching unit includes a
differential pair of transistors; transistors each having a collector or
drain connected in common to the emitters or sources of said transistors
and an emitter or source connected in common; and a current source
connected to the emitters or sources of said transistors; and a signal
voltage is input to the bases or gates of transistors in a differential
pair configuration and control voltages are input respectively to the
bases or gates of said transistors.
Moreover, the cascode transistors have collectors or drains connected in
common to the emitter or source of the cascode transistor; emitters or
drains connected respectively to the collectors or drains of the
transistors and to the resistors; and bases or drains connected in common
to a bias power source.
The filter circuit may further include a differential pair configuration
which consists of a current source connected to the emitters or sources of
the emitters or sources ground transistors via the resistors at the
juncture thereof; transistors; resistors; frequency band control circuits;
and cascade arranged transistors.
Furthermore, the filter circuit may further include the cascode transistors
having collectors or drains connected in common to the emitter or source
of the cascode transistor; emitters or sources connected in common to the
collector or drain of the common-emitter or common-source transistor and
to the resistor; and bases or gates connected in common to the bias power
source.
The filter circuit may further include a differential pair configuration
which consists of common-emitter or common-source transistor having
emitters or sources connected to the current source via the resistors; the
resistors; the frequency band control circuits; and the cascode
transistors.
Furthermore, the filter circuit may further include cascode transistors in
a differential pair configuration having collectors or drains connected in
common respectively to the emitters or sources of the cascode transistors
in a differential pair configuration, emitters or sources connected
respectively to the collectors or drains of the source ground transistors
and to the resistors in a differential pair configuration, and bases or
gates connected in common to the bias power source.
The frequency band control units may have a different cut-off order,
respectively.
Furthermore, control voltages may be input to the frequency band variable
circuit to activate plural signal transmission paths at the same time.
The frequency band variable circuits are cascoded to control independently
the respective frequency bands.
The frequency band control units are of different cut-off order,
respectively; and the control voltages input to the frequency band
variable circuit control activate simultaneously the plural signal
transmission paths.
The frequency band control units are a different cut-off order,
respectively; and the frequency-band variable circuits are cascoded and
controled independently respective frequency bands.
Moreover, the frequency band control units are of different cut-off order,
respectively; and the frequency-band variable circuits are cascoded to
input control voltages which activate simultaneously the plural signal
transmission paths to the control voltages added to the frequency band
variable circuit, respectively, whereby respective frequency bands are
independently controlled.
As described above, according to the present invention, the filter circuit
can constitute an active secondary low-pass filter circuit, an active
secondary high-pass filter circuit, an active band-pass filter circuit,
and an active resonance circuit, thus varying broadly its cut-off
frequency and resonance frequency.
Furthermore, the filter circuit according to the present invention uses the
input capacitance of a transistor of which the collector and the emitter
are grounded with high-frequency components. This feature can eliminate
the use of the discrete capacitor, thus realizing easily an integrated
filter circuit.
Hence, there is an advantage in that the entire system can be miniaturized
and the system performance can be largely improved.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. 1(a) and 1(b) are diagrams each explaining the first aspect of the
active filter according to the present invention;
FIGS. 2(a) and 2(b) are diagrams each explaining the second aspect of the
active filter according to the present invention;
FIGS. 3(a) and 3(b) are diagrams each explaining the third aspect of the
active filter according to the present invention;
FIGS. 4(a) and 4(b) are diagrams each explaining the fourth aspect of the
active filter according to the present invention;
FIGS. 5(a) and 5(b) are diagrams each explaining the fifth aspect of the
active filter according to the present invention;
FIGS. 6(a) and 6(b) are diagrams each explaining the sixth aspect of the
active filter according to the present invention;
FIGS. 7(a) and 7(b) are diagrams each explaining the seventh aspect of the
active filter according to the present invention;
FIGS. 8(a) and 8(b) are diagrams each explaining the eighth aspect of the
active filter according to the present invention;
FIG. 9 is a diagram explaining the active filter circuit according to the
first embodiment of the present invention;
FIG. 10 is a diagram explaining the active filter circuit according to the
second embodiment of the present invention;
FIG. 11 is a diagram explaining the active filter circuit according to the
third embodiment of the present invention;
FIG. 12 is a diagram explaining the active filter circuit according to the
fourth embodiment of the present invention;
FIG. 13 is a diagram explaining the active filter circuit according to the
fifth embodiment of the present invention;
FIG. 14 is a diagram showing the original configuration of the circuit
shown in FIG. 13;
FIG. 15 is a diagram explaining the active filter circuit according to the
sixth embodiment of the present invention;
FIG. 16 is a diagram explaining the active filter circuit according to the
seventh embodiment of the present invention;
FIG. 17 is a diagram explaining the active filter circuit according to the
eighth embodiment of the present invention;
FIG. 18 is a diagram explaining the active filter circuit according to the
ninth embodiment of the present invention;
FIG. 19 is a diagram explaining the active filter circuit according to the
tenth embodiment of the present invention;
FIG. 20 is a diagram explaining the active filter circuit according to the
eleventh embodiment of the present invention;
FIG. 21 is a diagram explaining the active filter circuit according to the
twelfth embodiment of the present invention;
FIG. 22 is a diagram explaining the active filter circuit according to the
thirteenth embodiment of the present invention;
FIG. 23 is a diagram showing an equivalent circuit to calculate the
impedance in expectation of the emitter;
FIG. 24 is a graph showing the first resultant simulation;
FIG. 25 is a graph showing the second resultant simulation;
FIG. 26 is a block diagram explaining the aspect of the frequency band
variable filter circuit according to the present invention;
FIG. 27 is a block diagram showing the frequency band variable filter
circuit according to the first embodiment of the present invention;
FIG. 28 is a block diagram showing the frequency band variable filter
circuit according to the second embodiment of the present invention;
FIG. 29 is a block diagram showing the frequency band variable filter
circuit according to the third embodiment of the present invention;
FIG. 30 is a block diagram showing the frequency band variable filter
circuit according to the fourth embodiment of the present invention;
FIG. 31 is a block diagram showing the frequency band variable filter
circuit according to the fifth embodiment of the present invention;
FIG. 32 is a block diagram showing the frequency band variable filter
circuit according to the sixth embodiment of the present invention;
FIG. 33 is a block diagram showing the frequency band variable filter
circuit according to the seventh embodiment of the present invention;
FIG. 34 is a block diagram showing the frequency band variable filter
circuit according to the eighth embodiment of the present invention;
FIG. 35 is a block diagram showing the frequency band variable filter
circuit according to the ninth embodiment of the present invention;
FIG. 36 is a block diagram showing the frequency band variable filter
circuit according to the tenth embodiment of the present invention;
FIGS. 37(a), 37(b), and 37(c) are diagrams each explaining the frequency
characteristic of the frequency band variable filter circuit according to
the eleventh embodiment of the present invention;
FIG. 38 is a diagram showing the frequency band variable filter circuit
according to the twelfth embodiment of the present invention;
FIGS. 39(a), 39(b), and 39(c) are diagrams each explaining the frequency
characteristic of the frequency band variable filter circuit according to
the twelfth embodiment of the present invention;
FIG. 40 is a diagram showing the concrete cascode connection configuration
of the embodiment shown in FIG. 38;
FIG. 41 is a diagram showing the frequency band variable filter according
to the second embodiment of the present invention;
FIG. 42 is a graph showing an example of the result simulated by means of
SPICE;
FIG. 43 is a diagram showing the frequency band variable filter according
to the fifth embodiment of the present invention;
FIG. 44 is a graph showing an example of the result that SPICE simulated
the circuit shown in FIG. 43;
FIG. 45 is a diagram showing the frequency band variable filter according
to the tenth embodiment of the present invention;
FIG. 46 is a graph showing an example of the result that SPICE simulated
the circuit shown in FIG. 45;
FIGS. 47(a) and 47(b) are diagrams each showing the structural example of a
current source;
FIG. 48 is a diagram showing the configuration of the optical signal
receiving unit in an optical communications system;
FIG. 49 is a diagram showing an example of a conventional active filter
circuit;
FIG. 50 is a diagram showing a first conventional frequency band variable
filter circuit;
FIG. 51 is a graph showing the result simulated by SPICE; and
FIG. 52 is a diagram showing a second conventional frequency band variable
filter circuit.
BEST MODE FOR CARRYING OUT THE INVENTION
The embodiments according to the present invention will be described below
with reference to the attached drawings.
(a) The fundamental explanation of the portion related to the active filter
circuit according to the present invention:
FIG. 1 is a diagram showing a first aspect of the present invention. FIG.
1(a) is a structural diagram showing the secondary low-pass filter
circuit. FIG. 1(b) is a diagram showing the equivalent circuit of the
secondary low-pass filter circuit.
Referring to FIG. 1(a), symbol T1 represents a transistor forming a coil,
R1 and R2 represent resistors, and C1 represents a capacitor. Now when it
is assumed that an input current is the current flowing into the emitter
of the transistor T1, the resistor R2, and the capacitor C1 and an output
current is the current derived from the collector of the transistor T1,
the current transfer function of the circuit shown in FIG. 1(a) is
calculated with reference to FIG. 23.
That is, FIG. 23 is a diagram showing an equivalent circuit used for
calculating the impedance counting the emitter of the transistor T1 shown
in FIG. 1(a). Referring to FIG. 23, R1 corresponds to the resistor R1
shown in FIG. 1(a) and Z.pi. is the base to emitter impedance of the
.pi.-type equivalent circuit of a transistor; gm is a transconductance of
the transistor, v is the terminal voltage of the impedance Z.pi., and
gm.multidot.V is a collector current source of the transistor.
Referring to the equivalent circuit shown in FIG. 23, the impedance
counting the emitter of the transistor is expressed as follows:
ZE=1/gm+R1(1/.beta.o+.OMEGA..sup.2)/(1+.OMEGA..sup.2)+j.OMEGA.(R1-1/gm)/(1+
.OMEGA..sup.2) (1)
where .OMEGA.=.omega./.omega.T, .omega.=2.pi.f(fT is a
frequency),.omega.T=2.pi.fT(fT is the transition frequency of a
transistor), .beta.o is a common-emitter current gain, and R1 is the
resistance value of the resistor R1. Hereinafter, the resistance of the
resistor RN is denoted as Rn and the capacitance of the capacitor CN is
denoted as Cn.
Now, by setting R1>>1/gm and applying 1>>1/.beta.o, usually applicable, to
the formula (1), the following approximate formula holds within the
frequency range of 1>>(.omega./.omega.T).sup.2 :
ZE.apprxeq.j.omega.R1/.omega.T (2)
Hence, the equivalent element forming ZE shown in FIG. 23 is an equivalent
inductance expressed by the following formula:
Leq R1/.omega.T (3)
The equivalent circuit shown in FIG. 1(b) is obtained by substituting the
equivalent inductance expressed by the formula (3) for the transistor T1
shown in FIG. 1(a).
The current transfer function (Iout/Iin) of the equivalent circuit shown in
FIG. 1(b) is expressed as follows:
Iout/Iin=(1/C.sub.1 Leq1)/›s.sup.2 +(1/C.sub.1 R2)s+1/C1Leq1!(4)
where Leq1=R1/.omega.T.
The cut-off frequency (.omega.o and Q of the low-pass filter circuit are
expressed by the following formulas:
.omega.o=1/C.sub.1.sup.1/2 Leq.sub.1.sup.1/2 (5)
Q=.omega.oR.sub.2 C1 (6)
In other words, the circuit shown in FIG. 1(a) is a low-pass filter circuit
having a secondary cut-off characteristic in which current is input and
output.
As described above, in the filter circuit according to the first aspect of
the present invention, the characteristic of a coil is realized by a
transistor. Hence a current input/output type LCR low-pass filter circuit
with a secondary cut-off characteristic can be constructed without using a
discrete coil (L) component. There is an advantage in that this feature
allows the current input/output type low-pass filter circuit to be
integrated very easily.
In the following description, the equivalent circuits shown in FIGS. 2(b)
to 8(b) correspond to circuits in which the transistors in the circuits
shown in FIGS. 2(a) to 8(a) are substituted for the equivalent
inductances.
FIG. 2 is a diagram showing the second aspect of the present invention.
FIG. 2(a) is a diagram showing the configuration of the secondary low-pass
filter circuit. FIG. 2(b) is a diagram showing the equivalent circuit.
In FIG. 2(a), symbol T7 represents a transistor forming an inductance, R10
and R11 represent resistors; C5 represents a capacitor, and CCS3
represents a constant current source.
In this case, the resistor R10 has one terminal connected to the base of
the transistor T7 and the other terminal connected to an input voltage and
the transistor T7 outputs its emitter voltage. By calculating with the
equivalent circuit shown in FIG. 2(b), the voltage transfer function
(Vout/Vin) of the circuit shown in FIG. 2(a) is expressed by the following
formula:
Vout/Vin=(1/C.sub.5 Leq.sub.2)/›s.sup.2 +(1/C.sub.5 R.sub.11)s+1/C.sub.5
Leq.sub.2 ! (7)
where Leq2=R10 /.omega.T.
The cut-off frequency .omega.o and Q of the low-pass filter circuit are
expressed by the following formulas:
.omega.o=1/C.sub.5.sup.1/2 Leq.sub.2.sup.1/2 (8)
Q=.omega.o R.sub.11 C.sub.5 (9)
The circuit shown in FIG. 2(a) is a current input/output type low-pass
filter circuit having a secondary cut-off frequency.
As described above, according to the filter circuit of the second aspect of
the present invention, since the coil characteristic is realized, like the
first aspect, using a transistor, a voltage input/output type LCR filter
circuit with a secondary cut-off characteristic can be formed without
using a discrete coil. Hence, a voltage input/output type secondary
low-pass filter can be integrated very easily in a monolithic form.
In the filter circuit shown in FIG. 2(a), the transistor T7 has the
collector connected to the power source VCC. However, it is possible to
make a secondary low-pass filter circuit by connecting a certain impedance
between the collector of the transistor T7 and the power source VCC. In
this case, the equivalent inductance is not R10/.omega.T at the frequency
at which the Miller capacitance of the transistor T7 is comparable with
the resistor R10. Hence it should be noted that the frequency band over
which the secondary low-pass filter circuit can be used is narrowed.
FIG. 3 is a diagram showing the third aspect of the present invention. FIG.
3(a) is a diagram showing the configuration of the secondary high-pass
filter circuit. FIG. 3(b) is a diagram showing the equivalent circuit of
the secondary high-pass filter circuit.
Referring to FIG. 3(a), symbol T2 represents an inductance forming
transistor, R3 and R4 represent resistors, and C2 represents a capacitor.
It is assumed that an input current is the sum of the current flowing
through both the emitter of the transistor T2 and the resistor R4 and the
output current is the current derived from the capacitor C2. In this case,
by calculating with the equivalent circuit shown in FIG. 3(b), the current
transfer function (Iout/Iin) of the equivalent circuit shown in FIG. 3(b)
is expressed by the following formula:
Iout/Iin=(1/C.sub.2)s.sup.2 /›s.sup.2 +(1/C.sub.2 R.sub.4)s+1/C.sub.2
Leq3!(10)
where Leq3=R3/.omega.T. The cut-off frequency .omega.o and Q of the
high-pass filter circuit are expressed by the following formulas:
.omega.o=1/C.sub.2.sup.1/2 Leq.sub.3.sup.1/2 (11)
Q=.omega.o R.sub.4 C.sub.2 (12)
In other words, the circuit shown in FIG. 3(a) is a current input/output
type high-pass filter circuit having a secondary cut-off characteristic.
As described above, in the filter circuit of the third aspect of the
present invention, a transistor can realize the characteristic of a coil.
Hence a current input/output type LCR high-pass filter circuit with a
secondary cut-off characteristic can be constituted without using a coil.
This feature allows a current input/output type secondary low-pass filter
of to be integrated very easily in a monolithic form.
In this case, it is possible to make a secondary low-pass filter circuit by
connecting a certain impedance between the collector of the transistor T2
and the power source or GND. However it should be noted that the effective
frequency band is narrowed.
FIG. 4 is a diagram showing the fourth aspect of the present invention.
FIG. 4(a) is a diagram showing the configuration of the secondary
high-pass filter circuit. FIG. 4(b) is a diagram showing the equivalent
circuit of the secondary high-pass filter circuit.
Referring to FIG. 4(a), symbol T8 represents a transistor forming an
inductance, R12 and R13 represent resistors, and C6 represents a
capacitor, and CCS4 represents a constant current source.
It is assumed that the capacitor C6 has one terminal at which a voltage is
input and the other terminal connected to the emitter of the transistor T8
and the transistor T8 outputs its emitter voltage. In this case, by
calculating with the equivalent circuit shown in FIG. 4(b), the voltage
transfer function (Vout/Vin) of the equivalent circuit shown in FIG. 4(a)
is expressed by the following formula:
Vout/Vin=(1/C.sub.6)s.sup.2 /›s.sup.2 +(1/C.sub.6 R.sub.13)s+1/C.sub.6
Leq.sub.4! (13)
where Leq4 =R12/.omega.T. The cut-off frequency .omega.o and Q of the
high-pass filter circuit are respectively expressed by the following
formulas:
.omega.o=1/C.sub.6.sup.1/2 Leq.sub.4.sup.1/2 (14)
Q=.omega.o R.sub.13 C.sub.6 (15)
That is, the circuit shown in FIG. 4(a) is a voltage input/output type
high-pass filter circuit having a secondary cut-off frequency.
As described above, in the filter circuit of the fourth aspect of the
present invention, a transistor can realize the characteristic of a coil.
Hence a voltage input/output type LCR high-pass filter circuit with a
secondary cut-off characteristic can be constituted without using a
discrete coil. This feature allows a voltage input/output type secondary
low-pass filter to be integrated very easily in a monolithic form.
In this case, a certain impedance may be connected between the collector of
the transistor T8 and the power source VCC. However it should be noted
that the effective frequency band is narrowed.
FIG. 5 is a diagram showing the fifth aspect of the present invention. FIG.
5(a) is a diagram showing the configuration of the secondary band-pass
filter. FIG. 5(b) is a diagram showing the equivalent circuit of the
secondary band-pass filter.
Referring to FIG. 5(a), symbol T3 represents a transistor forming an
inductance, R5 and R6 represent resistors, and C3 represents a capacitor.
It is assumed that an input current flows into the emitter of the
transistor T3 and the capacitor C3 and an output current flows out through
the resistor 6. In this case, by calculating with the equivalent circuit
shown in FIG. 5(b), the voltage transfer function (Iout/Iin) of the
circuit shown in FIG. 5(a) is expressed by the following formula:
Iout/Iin=(1/C.sub.3 R.sub.6)s/›s.sup.2 +(1/C.sub.3 R.sub.6)s+1/C.sub.3
Leq.sub.5! (16)
where Leq5=R5/.omega.T. The cut-off frequency .omega.o and Q of the
band-pass filter circuit are respectively expressed by the following
formulas:
.omega.o=1/C.sub.3.sup.1/2 Leq.sub.5.sup.1/2 (17)
Q=.omega.oR.sub.6 C.sub.3 (18)
That is, the circuit shown in FIG. 5(a) is a current input/output type
high-pass filter circuit having a secondary cut-off frequency.
As described above, in the filter circuit of the fifth aspect of the
present invention, a transistor can realize the characteristic of a coil.
Hence a current input/output type LCR band-pass filter circuit with a
secondary cut-off characteristic can be constituted without using a
discrete coil. Thus, a current input/output type secondary low-pass filter
can be integrated very easily in a monolithic form.
In this case, a certain impedance can be connected between the collector of
the transistor T3 and the power source VCC. However it should be noted
that the effective frequency band is narrowed.
FIG. 6 is a diagram showing the sixth aspect of the present invention. FIG.
6(a) is a diagram showing the configuration of the secondary band-pass
filter. FIG. 6(b) is a diagram showing the equivalent circuit of the
secondary band-pass filter.
Referring to FIG. 6(a), symbol T9 represents a transistor forming an
inductance, R14 and R15 represent resistors, C7 represents a capacitor;
and CCS5 represents a current source.
It is assumed that an input voltage is applied to the emitter of the
transistor T9 via the resistor R15 and an output voltage is output as an
emitter voltage. In this case, by using the equivalent circuit shown in
FIG. 6(b), the voltage transfer function (Vout/Vin) of the equivalent
circuit shown in FIG. 6(a) is expressed by the following formula:
Vout/Vin=(1/C.sub.7)s/›s.sup.2 +(1/C.sub.7 R.sub.15)s+1/C.sub.7 Leq.sub.6
!(19)
where Leq.sub.6 =R.sub.14 /.omega.T.
The cut-off frequency .omega.o and Q of the band-pass filter circuit are
expressed by the following formulas:
.omega.o=1/C.sub.7.sup.1/2 Leq.sub.6.sup.1/2 (20)
Q=.omega.oR.sub.15 C.sub.7 (21)
That is, the circuit shown in FIG. 6(a) is a voltage input/output type
band-pass filter circuit having a secondary cut-off frequency.
As described above, in the filter circuit of the sixth aspect of the
present invention, a transistor can realize the characteristic of a coil.
Hence a voltage input/output type LCR band-pass filter circuit with a
secondary cut-off characteristic can be constituted without using a
discrete coil component. This feature allows the voltage input/output type
secondary band-pass filter to be integrated very easily.
In this case, a certain impedance can be connected between the collector of
the transistor T9 and the power source VCC. However it should be noted
that the effective frequency band is narrowed.
FIG. 7 is a diagram for explaining the seventh aspect of the present
invention. FIG. 7(a) is a diagram showing a circuit forming a resonator.
FIG. 7(b) is a diagram showing the equivalent circuit of the resonator.
Referring to FIG. 7(a), symbol T4 represents a transistor forming an
inductance, R7 represents a resistor, and C4 represents a capacitor.
It is assumed that an input current is the sum of the current flowing into
the emitter of the transistor T4 and the capacitor C4 and an output
voltage is output as an emitter voltage of the transistor T4. In this
case, by using the equivalent circuit shown in FIG. 7(b), the voltage
transfer function (Vout/Vin) of the equivalent circuit shown in FIG. 7(a)
is expressed by the following formula:
Vout/Vin=(1/C.sub.4)s/›s.sup.2 +1/C.sub.4 Leq.sub.7 ! (22)
where Leq7=R7/.omega.T.
The resonance frequency .omega.o of the resonator is expressed by the
following formula:
.omega.o=1/C.sub.4.sup.1/2 Leq.sub.7.sup.1/2 (23)
That is, the circuit shown in FIG. 7(a) is a resonator in which current is
input and voltage is output.
As described above, in the filter circuit of the seventh aspect of the
present invention, a transistor can realize the characteristic of a coil.
Hence a current-input/ voltage-output type LCR resonator can be
constituted without using a discrete coil component. There is an advantage
in that the current-input/ voltage-output type resonator can be integrated
very easily in a monolithic form.
In this case, a certain impedance can be connected between the collector of
the transistor T4 and the power source VCC. However it should be noted
that the effective frequency band is narrowed.
FIG. 8 is a diagram showing the eighth aspect of the present invention.
FIG. 8(a) is a diagram showing a resonator forming circuit. FIG. 8(b) is a
diagram showing the equivalent circuit of the resonator.
Referring to FIG. 8(a), symbol T10 represents a transistor forming an
inductance, R16 represents a resistor, C8 represents a capacitor; and CCS6
represents a constant current source.
An input voltage is applied to the emitter of the transistor T10 via the
capacitor C8 and an output current is derived from the collector of the
transistor T10. In this case, by using the equivalent circuit shown in
FIG. 8(b), the voltage transfer function (Iout/Vin) of the equivalent
circuit shown in FIG. 8(a) is expressed by the following formula:
Iout/Vin=(1/Leq8)s/(s.sup.2 +1/C.sub.8 Leq.sub.8) (24)
where Leq.sub.8 =R.sub.16 /.omega.T. The resonance frequency .omega.o of
the resonator is expressed by the following formula:
.omega.o=1/C.sub.8.sup.1/2 Leq.sub.8.sup.1/2 (25)
That is, unlike the resonator shown in FIG. 7(a), the circuit shown in FIG.
8(a) is a voltage-input/ current-output type resonator.
As described above, in the filter circuit of the eighth aspect of the
present invention, a transistor can realize the characteristic of a coil.
Hence a voltage-input/ current-output type LCR resonator can be
constituted without using a discrete coil component. This feature allows
an integrated resonator to be manufactured very easily.
(b) Explanation of the Embodiment of the Active Filter Circuit:
FIG. 9 is a diagram showing the active filter circuit according to the
first embodiment of the present invention. Referring to FIG. 9, symbol T5
represents an input transistor, T6 represents an output transistor, and R8
and R9 represent resistors. The input terminal of the circuit shown in
FIG. 1(a), FIG. 3(a), or FIG. 5(a) is connected to the collector of the
transistor T5. The output terminal of the circuit shown in FIG. 1(a), FIG.
3(a), or FIG. 5(a) is connected to the emitter of the transistor T6.
If current Iin and Iout are ignored, Vin =Iin R8 and Vout=lout R9.
Hence, Vout/Vin=(R.sub.9 /R.sub.8)(Iout/Iin) (26)
The first, third and fifth aspects of the active filter circuit according
to the present invention correspond respectively to the secondary low-pass
filter circuit, the secondary high-pass filter circuit, and the secondary
band-pass filter circuit which are respectively expressed as the current
transfer functions of the formulas (5), (11) and (16). However, actual
filter circuits can be easily realized by converting each of the transfer
functions into a voltage transfer function. This feature enables the
down-sized system, in addition to a largely improved performance of the
system such as an optical communications system.
FIG. 10 is a diagram showing the active filter circuit according to the
second embodiment of the present invention. Referring to FIG. 10, symbol
T5 represents an input transistor, T4 represents a transistor forming an
inductance, R8 and R7 are resistors, and C4 represents a capacitor. The
circuit shown in FIG. 10 is formed of the transistor T5 having the
collector connected to the input terminal of the circuit shown in FIG.
7(a).
Like for the circuit shown in FIG. 9, since Vin=Iin R8, the following
formula is held:
Vout/Vin=(1/R.sub.8)(Vout/Iin) (27)
According to the active filter circuit of the second embodiment of the
present invention, in the resonator expressed as the transfer impedance
formula (2) in the seventh aspect thereof, the transfer function can be
converted into a voltage transfer function. This feature allows the
resonator to be easily realized as an actual circuit.
FIG. 11. is a diagram showing the active filter circuit according to the
third embodiment of the present invention.
Referring to FIG. 11, symbol T10 represents a transistor forming an
inductance, T11 represents an output transistor, R16 and R17 represent
resistors, C8 represents a capacitor, and CCS6 represents a constant
current source.
Since Vout=Iout R17, the following formula is held:
Vout/Vin=R.sub.17 Iout/Vin (28)
According to the active filter circuit of the third embodiment of the
present invention, the transfer admittance formula (22) for the resonator
can be converted into the above voltage transfer function in the eighth
aspect of the present invention. This feature the resonator to be easily
realized as an actual circuit.
FIG. 12 is a diagram showing the active filter circuit of the fourth
embodiment of the present invention. Referring to FIG. 12, T5a and T5b
represent input transistors, T6a and T6b represent output transistors,
R8a, R8b, R9a and R9b represent resistors, and CCS1a, CCS1b, and CCS3
represent constant current sources.
The input terminal of the circuit shown in FIG. 1(a), FIG. 3(a), or FIG.
5(a) is connected to the collectors of the transistors T5a and T5b. The
output terminal of the circuit shown in FIG. 1(a), FIG. 3(a), or FIG. 5(t)
are connected to the emitters of the transistors T6a and T6b.
In the circuit shown in FIG. 12, when the base voltage Vin of the
transistor T5a and the base voltage Vin* of the transistor T5b are equal
in amplitude but opposite in phase, the collector current of the
transistor T5a and the collector current of the transistor T5b are equal
in amplitude but opposite in phase while the collector voltages Vout of
the transistors T6a and T6b are equal in amplitude but opposite in phase.
Hence
(Vout-Vout*)/(Vin-Vin*)=(R.sub.9 /R.sub.8)(Iout/Iin) (29)
Like in the circuit shown in FIG. 9, the current transfer function is
converted into the voltage transfer function in the circuit shown in FIG.
12. Thus the secondary low-pass filter shown in FIG. 1, the secondary
high-pass filter shown in FIG. 3, and the secondary band-pass filter shown
in FIG. 5 can be easily realized as actual circuits. There is an advantage
in that the amplifier built in the optical communications system can be
largely improved in performance and the entire system can be miniaturized.
The circuit shown in FIG. 12 which has the bases of the transistors T5a and
T5b acting as input terminals and the collectors of the transistors T6a
and T6b acting as output terminals makes a balanced filter circuit. The
circuit which has the bases of the transistors T5a and T5b acting as input
terminals and the collector of the transistors T6a or T6b acting as output
terminals makes a balanced-input / unbalanced-output type filter circuit.
Moreover, with the base of the transistor T5a or T5b acting as an input
terminal and the base of the transistor T5b or T5a alternately grounded,
and the collector of the transistors T6a or T6b acting as an output
terminal, an unbalanced-input / balanced-output type filter circuit can be
formed. With one of the transistors T5a and T5b having the base acting as
an input terminal and the other of the transistor T5b and T5a having the
base alternately grounded, and the transistors T6a and T6b having the
collectors acting as output terminals, a balanced-input /
unbalanced-output type filter circuit can be formed.
With one of the transistors T5a and T5b having the base acting as an input
terminal and the other of the transistor T5b and T5a having the base
alternately grounded, and the transistors T6a and T6b having the
collectors acting as output terminals, an unbalanced/unbalanced-type
filter circuit can be formed.
The constant current sources CCS1a and CCS1b supply a bias current to the
inductance forming transistor and the output transistor arranged in the
black box within the circuit shown in FIG. 12 to enlarge the dynamic range
thereof.
FIG. 13 is a diagram used for explaining the active filter circuit
according to the fifth embodiment of the present invention. Referring to
FIG. 13, symbols T5a and T5b represent input transistors, T4a and T4b
represent transistors forming coils, R8a, R8b, R7a and R7b represent
resistors, C4' represents a capacitor, CCS1a, CCS1b and CCS2 represent
constant current sources.
The circuit shown in FIG. 13 corresponds to the circuit in which the input
terminal of the circuit shown in FIG. 7 is connected to the collectors of
the input transistors T5a and T5b. The original circuit configuration
resembles the circuit shown in FIG. 14. However, in consideration that the
input transistors T5a and T5b form a differential amplifier driven with
the constant current circuit CCS2, the circuit portion of the capacitors
C4a and C4b shown in FIG. 14 is modified.
For that reason, in the input transistors T5a and T5b shown in FIG. 14, the
ac components of the collector voltages are equal in amplitude and
opposite in phase.
Hence, current does not flow between the juncture between the capacitors
C4a and C4b and the ground. Even if the juncture is not ground, the
current distribution of the circuit does not change. Hence, the circuit of
the capacitors
C4a and C4b can be substituted for the capacitor C4' shown in FIG. 13.
If the total capacitance of the capacitors C4a and C4b shown in FIG. 14 is
C4 and the capacitance C4' shown in FIG. 13 has a capacitance of C.sub.4
/2, the voltage transfer function of the circuit shown in FIG. 13 has the
same frequency characteristic as that of the transfer function of the
circuit shown in FIG. 7.
The resonator shown in FIG. 7 can be easily realized as an actual circuit,
particularly, an integrated circuit.
Like the circuit shown in FIG. 12, the circuit shown in FIG. 13 can be used
for a balanced filter circuit, a balanced/unbalanced filter circuit, an
unbalanced/balanced filter circuit, or an unbalanced filter circuit. The
constant current sources CCS1a and CCS1b are used to expand the dynamic
range of the circuit shown in FIG. 13.
FIG. 15 is a diagram showing the active filter circuit according to the
sixth embodiment of the present invention.
Referring to FIG. 15, symbols T7a and T7b represent input and
inductance-forming transistors, R10a, R10b, and R11' represent resistors,
C5' represents a capacitor, and CCS3a and CCS3b represent constant current
sources.
The circuit shown in FIG. 15 includes two circuits each shown in FIG. 2.
The parallel circuit of the capacitor C5 and the resistor R11 shown in
FIG. 2 is modified by utilizing that the emitter voltages of respective
transistors are the same amplitude and in opposite phase and the collector
voltages of respective transistors are the same amplitude and in opposite
phase.
By selecting the capacitance of the capacitor C5' to 1/2of the capacitance
of the capacitor C5 and selecting the resistance value of the resistor
R11' to a value twice the resistance value of the resistor R11, the
voltage transfer function of the circuit shown in FIG. 15 has the same
frequency characteristic as that of the circuit shown in FIG. 2.
That is, according to the circuit shown in FIG. 15, a
voltage-input/voltage-output type secondary low-pass filter circuit can be
easily realized as an actual circuit, particularly, an integrated circuit.
As described above, according to the active filter circuit of the sixth
embodiment of the present invention, the secondary low-pass filter with no
coil can be easily realized as actual integrated circuit. Hence, the
performance of the amplifier used for the optical communications system
can be largely improved and the system can be effectively down-sized.
Like the circuit shown in FIG. 12, the circuit shown in FIG. 15 can be used
as a balanced filter circuit, a balanced/unbalanced filter circuit, an
unbalanced/balanced filter circuit, or an unbalanced filter circuit.
FIG. 16 is a diagram showing the active filter circuit according to the
seventh embodiment of the present invention. Referring to FIG. 16, symbols
T8a and T8b represent inductance-forming transistors, R12a, R12b and R13'
represent resistors, C6a and C6b represent capacitors, and CCS4a and CCS4b
represent constant current sources.
Like the modification to the circuit shown in FIG. 14, the circuit shown in
FIG. 16 is formed by using two circuits each shown in FIG. 4(a).
By selecting the resistance value of the resistor R13' to a value twice the
resistance value of the resistor R13, the voltage transfer function of the
circuit shown in FIG. 16 has the same frequency characteristic as that of
the voltage transfer function of the circuit shown in FIG. 4(a).
According to the circuit configuration shown in FIG. 16, the voltage-input
/ voltage-output type secondary high-pass filter circuit, shown in FIG. 4,
can be easily realized as an actual circuit.
As described above, according to the active filter circuit of the seventh
embodiment of the present invention, a secondary high-pass filter with no
coil can be easily realized as an actual circuit such as an integrated
circuit. Hence, the performance of the amplifier used for the optical
communications system can be largely improved and the system can be
effectively small-sized.
Like the circuit shown in FIG. 12, the circuit shown in FIG. 16 can be used
for a balanced filter circuit, a balanced/unbalanced filter circuit, an
unbalanced/balanced filter circuit, or an unbalanced filter circuit.
FIG. 17 is a diagram showing the active filter circuit according to the
eighth embodiment of the present invention. Referring to FIG. 17, symbols
T9a and T9b represent inductance forming transistors, R14a, R14b, R15a and
R15b represent resistors, C7' represents a capacitor, and CCS5a and CCS5b
represent constant current sources.
Like the modification to the circuit shown in FIG. 14, the circuit shown in
FIG. 17 is formed of two circuits shown in FIG. 6.
By selecting the capacitance of the capacitor C7' shown in FIG. 17 to a
half value of the capacitance of the capacitor C7 shown in FIG. 6, the
voltage transfer function of the circuit shown in FIG. 17 has the same
frequency characteristic as that of the voltage transfer function of the
circuit shown in FIG. 6.
According to the circuit configuration shown in FIG. 17, the
voltage-input/voltage-output type secondary band-pass filter circuit,
shown in FIG. 6, can be easily realized as an actual circuit.
As described above, according to the active filter circuit according to the
eighth embodiment of the present invention, a secondary band-pass filter
with no coil can be easily realized as an actual circuit such as an
integrated circuit. Hence, the performance of the amplifier used for the
optical communications system can be largely improved and the entire
system can be effectively small-sized.
Like the circuit shown in FIG. 12, the circuit shown in FIG. 17 can be used
for a balanced filter circuit, a balanced/unbalanced filter circuit, an
unbalanced/balanced filter circuit, or an unbalanced filter circuit.
FIG. 18 is a diagram showing the active filter circuit according to the
ninth embodiment of the present invention. Referring to FIG. 18, symbol T5
represents an input transistor, T1 represents an inductance forming
transistor, T12 represents a transistor which forms a resistor connected
to the base of the transistor T1, R8, R2, and R9 represent resistors, C1
represents a capacitor, CCS1 represents a constant current source, and
VCS1 represents a variable constant current source which supplies the
emitter current to the transistor T12.
The circuit shown in FIG. 18 corresponds to the circuit shown in FIG. 1
arranged in the black-box shown in FIG. 9. The resistor R1 in FIG. 1 is
substituted for the emitter resistor r.sub.e of the transistor T12 in FIG.
18.
When the emitter current is Ie (mA), the emitter resistor r.sub.e is
expressed as follows:
r.sub.e =V.sub.T /Ie(here, V.sub.T =26mV) (30)
r.sub.e of the formula (30) is substituted for the formula (4) and then the
result is substituted for the formula (5). Thus the resultant frequency
characteristic is the same as that of the circuit shown in FIG. 18.
As a result, in the circuit shown in FIG. 18, the resistor R1 of a fixed
value in the circuit shown in FIG. 1 can be varied with the emitter
current of the transistor T12. That is, according to the circuit shown in
FIG. 18, a low-pass filter circuit with a secondary cut-off characteristic
and a cut-off frequency capability can be realized as an actual circuit by
varying the emitter current of the transistor T12 by means of the variable
current source VCS1.
As described above, according to the active filter circuit of the ninth
embodiment of the present invention, a secondary low-pass filter which can
vary the cut-off frequency can be easily realized as an actual circuit
such as an integrated circuit. Hence, the performance of the amplifier
used for the optical communications system can be largely improved and the
system can be effectively small-sized.
As for the circuit shown in FIG. 18 embodying the first aspect of the
present invention, the resistor connected to the base of the transistor
forming an inductance is substituted for the emitter resistor r.sub.e of
the transistor driven by means of the variable current source. Similar
substitution can be applied to the active filter circuit according to the
third to eighth aspects of the present invention.
FIG. 19 is a diagram showing the active filter circuit according to the
tenth embodiment of the present invention. Referring to FIG. 19, T13
represents a transistor which functions as an input transistor and a
resistor forming transistor connected to the base of the transistor T7 (to
be described later), T7 represents a transistor forming an inductance, R11
represents a resistor, C5 represents a capacitor, CCS3 represents a
constant current source, and VCS2 represents a variable current source.
In the circuit shown in FIG. 19, the emitter resistor of the transistor T13
is substituted for the resistor R10 in the circuit shown in FIG. 2.
Since the transistor T13 acts as an emitter follower stage for amplifying
the signal, the ac base potential of the transistor T13 equals to that of
the transistor T7. The frequency characteristic of the circuit shown in
FIG. 19 is similar to that of the circuit shown in FIG. 2.
The circuit shown in FIG. 19 is a low-pass filter with a secondary cut-off
characteristic. The cut-off frequency can be varied with the current from
the variable current source VCS2.
As described above, according to the active filter circuit of the tenth
embodiment of the present invention, the secondary low-pass filter circuit
which can vary the cut-off frequency, like the ninth embodiment, can be
easily realized as an actual circuit, particularly, an integrated circuit.
Hence, a cut-off frequency range controllable low-pass filter circuit can
be realized as an integrated circuit. The performance of the amplifier
used for the optical communications system can be largely improved and the
system can be effectively small-sized.
FIGS. 18 and 19 show a signal-input and single-output type circuit.
However, the substitution to a variable resistor can be applied to the
differential amplifier-type circuit.
FIG. 20 is a diagram showing the active filter circuit according to the
eleventh embodiment of the present invention. Referring to FIG. 20, T5
represents an input transistor, T1 represents a transistor forming an
inductance, T6 represents an output transistor, T1 represents a transistor
forming a capacitance, R.sub.8, R.sub.1, R.sub.9 and R.sub.2 represent
resistors, C9 represents a bypass capacitor which ac-grounds the emitter
of the transistor T14, CCS1 represents a constant current source expanding
the dynamic range, and VCS3 represents a variable current source which
varies the emitter current.
The circuit shown in FIG. 20 can be formed by inserting the circuit shown
in FIG. 1 in the black-box shown in FIG. 9. The capacitor C1 shown in FIG.
1 is substituted for the input capacitance of the transistor T14 in FIG.
20.
Since the frequency characteristic is the same as that expressed by the
formula (5), the circuit shown in FIG. 20 is a low-pass filter with a
secondary cut-off characteristic.
As shown in FIG. 20, the transistor T14 has the input capacitance of
(Cd+CJE+CJC) because the collector and emitter thereof are grounded
alternatingly.
Cd>>CJE, CJC at a normal bias. Since depending on the electric charges
injected from the emitter into the base and the emitter to base voltage,
the emitter diffusion capacitance Cd is proportional to the emitter
current.
Hence, since the input capacitance of the transistor T14 varies
substantially and proportionally the current from the variable current
source VCS3, the secondary low-pass filter shown in FIG. 20 can vary its
cut-off frequency with the current from the variable current source VCS3.
According to the active filter circuit of the eleventh embodiment of the
present invention, the capacitance of the capacitor can be realized with
the input capacitance of the transistor grounded with high-frequency.
Hence the secondary low-pass filter circuit which can vary the cut-off
frequency can be easily realized as an actual circuit, particularly, an
integrated circuit without using a discrete coil. A cut-off frequency
controllable low-pass filter circuit can be realized as an integrated
circuit. Thus, the performance of the amplifier arranged in the optical
communications system is more improved and the entire system can be
effectively small-sized.
The circuit shown in FIG. 20 is a single-input and single-output type
circuit. Substituting the above-mentioned capacitor for a variable
capacitor can be applied to the differential amplifier-type circuit.
FIG. 21 is a diagram showing the active filter circuit according to the
twelfth embodiment of the present invention. Referring to FIG. 21, T5a and
T5b represent input transistors, T1a and T1b represent transistors forming
inductances, T6a and T6b represent output transistors, T15a and T15b
represent transistors forming variable capacitors, R8a, R8b, R2', R1a,
R1b, R9a and R9b represent resistors, CCS2 represents a constant current
source, CCS1a and CCS1b represent constant current sources each expanding
the dynamic range, and VCS3 represents a variable current source each
which varies the emitter current.
The circuit, shown in FIG. 21, is formed by applying the circuit shown in
FIG. 1 to the black-box shown in FIG. 12. The capacitor C1 shown in FIG. 1
is substituted for the input capacitance of each of the transistors T15a
and T15b.
The circuit shown in FIG. 21 has the same transfer function as that of the
circuit shown in FIG. 1. The circuit shown in FIG. 21 is a low-pass filter
having a secondary cut-off characteristic.
As shown in FIG. 21, the input capacitance of each of the transistors T15a
and T15a is the sum of (Cd+CJE+CJC) because the juncture between the
emitters of the transistors T15a and T15b is virtually grounded. Hence,
the secondary low-pass filter circuit shown in FIG. 21 can vary the
cut-off frequency in accordance with the current from the variable current
source VCS3.
According to the active filter circuit of the twelfth embodiment of the
present invention, the secondary low-pass filter circuit which can vary
the cut-off frequency can be easily realized as an actual circuit,
particularly, an integrated circuit without using a discrete coil. Hence a
low-pass filter circuit which can control the cut-off frequency can be
realized as an integrated circuit. Thus, the performance of the amplifier
arranged in the optical communications system is more improved and the
entire system can be effectively small-sized.
According to the present embodiment, as described above, a pair of the
active filter circuits embodying the first aspect of the present invention
forms, as an example, a differential amplifying circuit. However, the
method for varying a capacitance with a transistor can be applied to the
circuit according to other aspects but should not be limited to the mode
of the differential amplifier.
FIG. 22 is a diagram showing the active filter circuit according to the
thirteenth embodiment of the present invention. Referring to FIG. 22, T5a
and T5b represent input transistors, T1a and T1b represent transistors
forming inductances, T16a and T16b represent transistors having the
emitter resistors acting as resistors, T6a and T6b represent output
transistors; R8a, R8b, R2', R1a, R1b, R9a and R9b represent resistors, C1'
and C1" represent capacitors, CCS2 represents a constant current source,
and CCS1a and CCS1b represent constant current sources each expanding the
dynamic range.
The circuit of the transistors T5a, T1a and T6a (or of the transistors T5b,
T16b and T6b ) corresponds to the circuit in the black box shown in FIG. 1
to form a low pass filter circuit with a secondary cut-off frequency.
The circuit of the transistors T5a, T16a and T6a (or of the transistors
T5b, T1b and T6b ) corresponds basically to a low pass filter circuit with
a first cut-off frequency characteristic shown in FIG. 49.
The circuit shown in FIG. 22 can switch the secondary low-pass filter
circuit and the primary low-pass filter circuit by selectively setting the
base bias voltage V2 for the transistors T1a and T1b and the base bias
voltage V1 for the transistors T16a and T16b. In the circuit shown in FIG.
22, the secondary low-pass filter circuit is selected by flowing current
through the transistors T1a and T1b, with the base bias voltage V1 set to
a low value, the transistors T16a and T16b cut off, and the base bias
voltage V2 set to a high value. On the contrary, the primary low-pass
filter circuit is selected by supplying current through the transistors
T16a and T16b, with the transistors T1a and T1b cut off.
As described above, the active filter circuit of the thirteenth embodiment
of the present invention allows the same integrated circuit to have
different functions, together with the effect of the secondary low-pass
filter circuit. Hence there is an advantage in that the circuit designing
flexibility can be improved.
In the circuit shown in FIG. 22, the resistors R1a and R1b can be replaced
by the emitter resistance r.sub.e of the transistor to which current is
supplied from the variable current source. The capacitors C1' and C1" can
be replaced by the capacitance counting the base capacitance of the
transistor to which current is supplied from the variable current source.
According to the present embodiment, the circuit where the primary low-pass
filter circuit and the secondary low-pass filter circuit are combined and
separated has been described as an example. However, the filter circuits
combined should not be limited only to the above embodiment. Filter
circuits with arbitrary characteristics can be combined. The filter
circuit also should not be limited to the differential amplifier-type
filter.
FIGS. 24 and 25 are graphs each showing the result simulated by using SPICE
(circuit simulator). FIG. 24 shows the characteristic of the high-pass
filter circuit in which the resistor R12 of the circuit shown in FIG. 4(a)
is replaced by the emitter resistor r.sub.e of the transistor to which
current is supplied from the variable current source. As shown in FIG. 24,
the cutoff frequency increases (because of the decreasing resistance of
r.sub.e) as the current from the variable current source increases.
FIG. 25 shows the characteristic of the low-pass filter circuit shown in
FIG. 21. As shown in FIG. 25, the cut-off frequency decreases (because Cd
increases proportionally) as the emitter current of the variable
capacitance forming transistor increases.
In the above-mentioned embodiments, active filter circuits including
bipolar transistors (referred to as normal transistors) acting as active
elements have been described. Similar circuits having the basic operation
can be formed using field-effect transistors.
The reason is that the impedance viewed from the source of the field effect
transistor which has the gate grounded via a resistor can be expressed
with the equivalent circuit shown in FIG. 23.
Because of a relatively low value of gm, it cannot be avoided that the
filter circuit formed of field-effect transistors is inferior in the
characteristic to the filter circuit formed of transistors. However, there
is an advantage in that the filter circuit can be very easily formed with
digital circuits on the same chip.
(c) Basic Explanation of the Frequency-band Variable Filter Circuit
according to the Invention:
FIG. 26 is a block diagram showing the frequency range variable filter
circuit according to an aspect of the present invention. Referring to FIG.
26, numeral 21 represents a signal path switching unit; 51, 52, . . . , 5N
represent frequency range control units; 401 represents a cascode
transistor, 801 represents a resistor, VB1 is a bias power source, and a,
b, . . . , and n are signal transmission lines.
FIG. 26 shows the frequency band variable filter circuit is used in the
signal receiving unit arranged in the optical communications system. The
frequency band variable filter circuit consists of a signal path switching
unit 21 which switches an input signal to the signal transmission line a,
b, . . . or n by changing the applied control voltage; frequency band
control units 51, 52, . . . , and 5N connected respectively to the signal
transmission lines a, b, . . . , and n and each having a different range
of bandwidth; and cascode transistor 401 having the emitter connected to
the signal path switching unit 21 via the frequency range control units
51, 52, . . . , and 5N.
In order to achieve the purpose, the input signal transmission path is
switched by changing the control voltage applied to the signal path
switching unit 21 so that the frequency range of each of the signal
transmission lines a, b, . . . , and n is controlled.
According to the first method which realizes the above-mentioned frequency
range variable filter circuit, the signal path switching unit 21 is formed
of a common-emitter transistor 101 having the common-emitter via the
resistor 901, and transistors 201, 202, . . . , 20N each having the
collector connected to the common-emitter transistors 101.
The frequency range control units 51, 52, . . . , and 5N are formed of
frequency range control circuits 501, 502, . . . , and 50N connected
respectively to the collectors of the transistors 201, 202, . . . , and
20N, and resistors having different impedances R2, . . . RN connected
respectively to the collectors of the transistors 202, . . . , and 20N.
The cascode transistor 401 has the emitter connected to the collector of
the common-emitter transistor 101 and respectively to the collectors of
the transistors 202, . . . , and 20N via the resistors 302, . . . , and
30N.
The transmission path which flows the collector signal current of the
common-emitter transistor 101 is switched by inputting the signal voltage
Vin to the base of the common-emitter transistor 101 and the control
voltages V1, V2, . . . , and VN to the bases of the transistors 201, 202,
. . . , and 20N. Thus the frequency range variable circuits 501, 502, . .
. , and 50N are selectively controlled to a signal current path.
In the first method, the signal path switching unit is formed of the
common-emitter transistor 101 grounded via the resistor 901; the
transistors 201, 202, . . . , and 20N; resistors 302, . . . , and 30N; the
frequency range control units 501, 502, . . . , and 50N, and the cascode
transistor 401. However, according to the second method, the signal path
switching unit can be formed of the differential pair configuration which
includes common-emitter transistors 101a and 101b having the emitters
connected respectively to the resistors 901a and 901b; a current source
701 connected to the juncture of the resistors 901a and 901b; transistors
201a, 201b, . . . , 20Na, and 20Nb; resistors 302a and 302b, . . . , 30Na,
and 30Nb; frequency range control circuits 501, 502, . . . , and 5ON; and
cascade-arranged transistors 401a and 401b.
Moreover, according to the third method, the signal path switching unit 21
is formed of the emitter ground transistor 101 grounded via the resistor
901; and the emitter ground transistors 102, . . . , and 10N grounded via
the resistors 902, . . . , and 90N.
In order to control the frequency range control circuits 501, 502, . . . ,
and SON, the signal path is switched by applying the signal voltage Vin
and the control voltages V1, V2, . . . , and VN to the bases of the
emitter ground transistors 101, 102, . . . , and 10N.
The signal path switching unit consists of the emitter ground transistors
101, 102, . . . , and 10N; the resistors 302, . . . , and 30N; the
frequency range control circuits 501, 502, . . . , and 5ON; and the
cascode transistor 401. However, according to the fourth method, the
signal path switching unit can be formed of the differential pair
configuration which includes the emitter ground transistors 101a, 101b,
102a, 102b, . . . , 10Na, and 10Nb connected respectively to the constant
current source 701 via the resistors 901a, 902a, . . . , 90Na, and 901b,
902b, . . . , and 90Nb; the resistors 302a, 302b, . . . , 30Na, and 30Nb;
the frequency range control circuits 501, 502, . . . , and 50N; and the
cascode transistors 401a and 401b.
The signal path switching unit 21 is formed of the emitter ground
transistors 101a, 102a, . . . , 10Na, and 101b, 102b, . . . , 10Nb.
However, according to the fifth method, the signal path switching unit can
be formed of a differential pair configuration which includes emitter
ground transistors 10a, 102a, . . . , 10Na, and 101b, 102b, . . . , 10Nb;
the transistors 201, 202, . . . , and 20N having the collectors connected
in common to the emitters of the transistors 110a, 101b, 102a, 102b, . . .
, 10Na, and 10Nb and the emitters connected in common; and the current
source 701 connected in common to the emitters of the transistors 201,
202, . . . , and 20N.
The signal voltage Vin is applied to the bases of the differential
transistor pairs 101a and 101b, 102a and 102b, . . . , and 10Na and 10Nb.
The control voltages V1, V2, . . , and VN are applied respectively to the
bases of the transistors 201, 202, . . . , and 20N. In order to control a
specific one among the frequency range control circuits 501, 502, . . . ,
and 50N, the signal path is switched by setting the control voltages V1,
V2, . . . and VN.
According to the sixth method, the system includes the transistors 401',
402', . . . , and 40N' have the collectors connected in common to the
emitter of the transistor 401, the emitters connected in common to the
collectors of the transistor 201 and to the resistors 302, . . . , and
30N, and the bases connected to the bias power source VB2.
The signal transmission path switching unit is formed of the emitter ground
transistor 101 grounded via the resistor 901; the transistors 201, 202, .
. . , and 20N; the resistors 302, . . . , and 30N; the frequency range
control circuits 501, 502, . . . , and 50N; and the cascade-arranged
transistors 401, 401', 402', . . . , and 40N'. However, according to the
seventh method, the signal transmission path switching unit can be formed
of the differential pair configuration which includes the emitter ground
transistors 101a and 101b grounded respectively via the resistors 901a and
901b; the constant current source 701 connected to the juncture between
the resistors 901a and 901b; the transistors 201a, 201b, 202a, 202b, . . .
, 20Na, and 20Nb; the resistors 302a, 302b, . . . , 30Na, and 30Nb; the
frequency range control circuits 501, 502, . . . , and 50N; and the
cascode transistors 401a, 401b, 401a', 401b', . . . , 40Na', and 40Nb'.
In the eighth embodiment, the third embodiment may be modified in such a
manner the cascode transistors 401', 402', . . . , and 40N' have
collectors connected in common to the emitter of the cascode transistor
401, the emitters connected to the collector of the emitter grounded
transistor 101 and the resistors 302, . . . , and 30N, respectively, and
the base connected in common to the bias power source VB2.
In the eighth embodiment, the signal transmission path switching unit
includes the emitter ground transistors 101, 102, . . . , and 10N, the
resistors 302, . . . , and 30N, the frequency range control circuits 501,
502, . . . , and 50N, and the cascade-arranged transistors 401, 401',
402', . . . , and 40N'. In the ninth embodiment. However, in the ninth
embodiment, the signal transmission path switching unit may be formed of a
differential pair configuration which includes the emitter ground
transistors 101a, 102a, . . . , 10Na, and 101b, 102b, . . . , 10Nb having
the emitters connected to the constant current source 701 via the
resistors 901a, 901b, 902a, 902b, . . . , 90Na, and 90Nb; the resistors
302a, 302b, . . . , 30Na, and 30Nb; the frequency range control circuits
501, 502, . . . , and 5ON; the cascade-arranged transistors 401a, 401b,
401a', 401b', 402a', 402b', . . . , 40Na', and 40Nb'.
According to the tenth method, the fifth method may be modified in such a
manner that the cascade transistors 401a', 401b', 402a', 402b', . . . ,
40Na', and 40Nb', have the collectors connected in common to the emitter
of the cascade transistors 401a and 401b, the emitters connected to the
collector of the emitter grounded transistors 101a and 101b and to the
resistors 302a, . . . , and 30Na, 302b, . . . , and 30Nb, respectively,
and the bases connected in common to the bias power source VB2.
According to the eleventh method, the first to tenth methods may be
modified in such a manner the control voltages V1, V2, . . . , and VN may
be applied to the frequency band variable filter circuit while plural
signal transmission paths can be activated simultaneously.
According to the twelfth method, the first to tenth methods may be modified
in such a manner that the frequency band variable filter circuits may be
cascaded so as to control independently the frequency ranges thereof.
According to the thirteenth method, the first to tenth methods may be
modified in such a manner that the control voltages V1, V2, . . . , and VN
are applied to the frequency band variable filter circuits so as to
activate simultaneously plural signal transmission paths while the
frequency band variable filter circuits may be cascaded so as to control
independently the frequency ranges thereof.
According to the fourteenth method, in the first to tenth methods, the
frequency range control units (51, 52, . . . , and 5N) may act as
frequency range control units (51, 52, . . . , and 5N) each with a
different cut-off frequency order.
According to the fifteenth method, in the first to tenth methods, the
frequency range control units (51, 52, . . . , and 5N) may act as
frequency range control units (51, 52, . . . , and 5N) each with a
different order cut-off characteristic while the control voltages V1, V2,
. . . , and VN may be applied to the frequency range variable filter
circuit so as to activate simultaneously plural signal transmission paths.
According to the sixteenth method, in the first to tenth methods, the
frequency range control units (51, 52, . . . , and 5N) may act as
frequency range control units (51, 52, . . . , and 5N) each with a
different order cut-off characteristic while the frequency band variable
filter circuits may be cascaded so as to control independently the
frequency ranges thereof.
According to the seventeenth method, in the first to tenth methods, the
frequency range control units (51, 52, . . . , and 5N) may act as
frequency range control units (51, 52, . . . , and 5N) each with a
different cut-off frequency order while the frequency band variable filter
circuits are cascaded so as to activate simultaneously plural signal
transmission paths, whereby the frequency ranges of the control voltages
V1, V2, and VN are independently controlled.
Next, the function of the frequency variable band filter circuit according
to the present invention will be described below.
According to the aspect of the frequency variable band filter circuit of
the invention described with FIG. 26, an input signal is output from the
cascade transistor 401 via the signal path switching unit 21 which
switches the signal transmission paths a, b, . . . , and n by changing the
applied control signal and one of the signal transmission paths a, b, . .
. , and n connected to the frequency range control units 51, 52, . . . ,
and 5N each having a different frequency range variable width. To pass an
input signal, the signal transmission paths a, b, . . . , and n can be
switched by varying the control voltage, thus controlling the frequency
range to an input signal.
According to the first method which realizes the aspect of the invention,
the signal path switching unit 21 is formed of the emitter ground
transistor having the emitter grounded via the resistor 901, and the
transistors 201, 202, . . . , and 20N connected to the collector of the
common-emitter transistor 101.
The frequency range control units 51, 52, . . . , and 5N is formed of the
frequency range control circuits 501, 502, . . . , and 50N connected to
the collectors to the transistors 201, 202, . . . , and 20N, and the
resistors 302, . . . , and 30N with different impedances R2, . . . , and
RN connected respectively to the collectors of the transistors 202, . . .
, and 20N.
The same effect can be obtained by using diodes, instead of the resistors
302, . . . , and 30N. The replacement are applicable to all the aspect of
the present invention. Hence, the diodes used instead of the resistors
302, . . . , and 30N will be omitted in the following explanation.
The cascade transistor 401 has the emitter connected to the collector of
the common-emitter transistor 101 and any one of the collectors to the
transistors 202, . . . , and 20N via the resistors 302, . . , and 30N.
In such a configuration, the signal voltage Vin is input to the base of the
common-emitter transistor 101 and the control voltages V1, V2, . . . , and
VN are input to the bases of the transistors 201, 202, . . . , and 20N.
When the control voltages V1, V2, . . . , and VN are controlled, a suitable
path with a desired frequency range variable width through which the
collector-signal current of the common-emitter transistor flows can be
selected among the signal transmission paths a, b, . . . , and n.
In the first method, the signal transmission path switching unit includes
the common-emitter transistor 101 having the emitter grounded via the
resistor 901, the transistors 201, 202, . . . , and 20N, the resistors
302, . . . , and 30N, the frequency range control circuits 501, 502, . . .
, and 50N, and the transistor 401. However, according to the second method
realizing the aspect of the present invention, the signal transmission
path switching unit can be formed of a differential pair configuration
which includes the common-emitter transistors 101a and 101b having the
emitters grounded via the resistors 901a and 901b; the transistors 201,
202, . . . , and 20N; the resistors 302a, 302b, . . . , 30Na, and 30Nb;
the frequency range control circuits 501, 502, . . . , and 50N; and the
cascade transistors 401a and 401b.
The input signal Vin is input to the base of the common-emitter transistor
101a and the inverted input signal Vin is input to the base of the
common-emitter transistor 101b. The control signals V1, V2, . . . , and VN
are input to the transistors 201a, 202a, . . . , 20Na, . . . , 201b, 202b,
. . . , 20Nb, respectively.
The collector signal current of the common-emitter transistor 101a and the
inverted signal of the common-emitter transistor 101b can be switched to
the optimum signal transmission path with a desired frequency range
variable width by controlling the control signals V1, V2, . . . , and VN.
In the third method realizing the aspect of the present invention, the
signal path switching unit 21 is formed of the common-emitter transistor
101 having the emitter grounded via the resistor 901 and the
common-emitter transistors 102, . . . , and 10N grounded via the resistors
902, . . . , and 90N.
The signal voltage Vin plus each of the control voltages V1, V2, . . . ,
and VN are input respectively to the base of the common-emitter
transistors 101, 102, . . . , and 10N.
The transmission path is switched to the optimum path having a desired
frequency range variable width by controlling the control voltages V1, V2,
. . . , and VN.
In the third method, the signal transmission path switching unit includes
the common-emitter transistors 101, 102, . . . , and ION, the resistors
302, . . . , and 30N, the frequency range control circuits 501, 502, . . .
, and 50N, and the cascade transistor 401. According to the fourth method
realizing the aspect of the present invention, the signal transmission
path switching unit can be formed of a differential pair configuration
which includes the common-emitter transistors 101a, 101b, 102a, 102b, . .
. , 10Na, and 10N having the emitters connected to the constant current
source 701; the transistors 302a, 302b, . . . , 30Na, and 30Nb; the
resistors 302a, 302b, . . . , 30Na, and 30Nb; the frequency range control
circuits 501, 502, . . . , and 50N; and the cascade transistors 401a and
401b.
The input signal and the control signal are superimposed to input to the
common-emitter transistors 101a, 102a, . . . , 10Na, 101b, 102b, . . . ,
and 10Nb.
The transmission path is switched to the optimum path having a desired
frequency range variable width by controlling the control voltages V1, V2,
. . . , and VN.
In the fifth method realizing the aspect of the present invention, the
signal path switching unit 21 formed of the common-emitter transistors
101a, 102a, . . . , 10Na, and 101b, 102b, . . . , and 10Nb is formed of
the differential transistor pairs 101a, 101b, 102a, 102b, . . . , 10Na and
10Nb; the transistors 201, 202, . . . , and 20N having the collectors
connected in common to the emitters of the transistors 101a, 101b, 102a,
102b, . . . , 10Na, and 10Nb and the emitter connected to the transistors
201, 202, . . . , and 20N; and the current source 701 connected to the
emitters of the transistors 201, 202, . . . , and 20N.
The input voltage Vin is input in common to the bases of the transistors
101a, 101b, 102a, 102b, . . . , 10Na, and 10Nb. The control voltages V1,
V2, . . . , and VN are respectively input to the bases of the transistors
201, 202, . . . , and 20N.
The transmission path can be switched to the optimum path with a desired
frequency range variable width by setting the control voltages V1, V2, . .
. , and VN.
According to the first method, the cascade transistors 401', 402', . . . ,
and 40N' has respectively the collectors connected in common to the
emitter of the cascade transistor 401, the emitters connected to the
collector of the transistor 201 and to the resistors 302, . . . , and 30N
in the other signal transmission paths, the bases connected to the bias
power source VB2. Hence, according to the sixth method realizing the
aspect of the present invention, like the first embodiment of the
invention, the transmission path can be switched to the optimum path with
a desired frequency range variable width.
According to the sixth method, like the differential pair configuration in
the second method applied to the first method, the differential pair
configuration is formed of the transistors 101, 201, 202, . . . , 20N,
401, 401', 402', . . . , and 40N'. Hence according to the seventh method
realizing the present invention, like the second method, the transmission
path is switched to the optimum path with a desired frequency range
variable width by setting the control voltages V1, V2, . . . , and VN.
According to the third method, the cascade transistors 401', 402', . . . ,
and 40N' have the collectors connected in common to the emitter of the
transistor 401, the emitters connected to the collector of the
common-emitter transistor 101 and to the resistors 302, . . . , and 30N,
and the bases connected in common to the bias power source VB2. Hence
according to the eighth method realizing the present invention, like the
third method, the transmission path can be switched to the optimum path
with a desired frequency range variable width by setting the control
voltages V1, V2, . . . , and VN.
According to the eighth method, like the differential configuration
relating to the fourth method applied to the third method, the
differential pair configuration is formed of the transistors 101, 102, . .
. , 10N, 401, 401', 402', . . . , and 40N'. Hence according to the ninth
method realizing the present invention, the transmission path can be
switched to the optimum path with a desired frequency range variable width
by setting the control voltages V1, V2, . . . , and VN.
According to the fifth method, the cascade transistors 401a', 401b', 402a',
402b', . . . , 40Na', and 40Nb', have respectively the collectors
connected in common to the emitters of the cascade transistors 401a and
401b, the emitters connected to the collector of the common-emitter
transistors 101a and 101b and to the resistors 302a and 302b, and the
bases connected to the bias power source VB2. Hence, according to the
tenth method realizing the aspect of the present invention, like the fifth
invention, the transmission path can be switched to the optimum path with
a desired frequency range variable width.
According to the first to tenth methods, the control voltages V1, V2, . . .
, and VN are input to activate simultaneously the plural signal
transmission paths. Hence, according to the eleventh method realizing the
aspect of the present invention, the frequency range can be controlled by
controlling the signal current ratio between plural transmission paths
operated on the control voltages V1, V2, . . . , and VN, without
controlling the frequency range control circuits 501, 502, . . . , and
50N.
According to all the first to tenth methods, the frequency range control
circuits are cascaded to control independently the frequency range
thereof. Hence, according to the twelfth method realizing the aspect of
the present invention, like the first to tenth methods, the transmission
path can be switched to the optimum path to vary the frequency range
thereof.
According to all the first to tenth methods, a voltage is added
respectively to the control voltages V1, V2, . . . , and VN so as to
operate simultaneously the plural signal transmission paths. The frequency
range variable circuits are cascaded to control independently their
frequency ranges. Hence, according to the thirteenth method realizing the
aspect of the present invention, the transmission path can be switched
selectively to the optimum path to control and vary the frequency range
thereof, whereby the frequency characteristics with a more-squared
shoulder can be obtained.
According to all the first to tenth methods, the frequency range control
units (51, 52, . . . , 5N) can act as frequency range control units (51,
52, . . . , and 5N) having different cut-off orders.
Hence, according to the fourteenth method realizing the aspect of the
present invention, the frequency range control units (51, 52, . . . , and
5N) can be switched to a desired cut-off order, respectively.
According to all the first to tenth methods, the frequency range control
units (51, 52, . . . , and 5N) can act as the frequency range control
units (51, 52, . . . , and 5N) with different cut-off orders and a voltage
is added to the control voltages V1, V2, . . . , and VN to activate
simultaneously plural signal transmission paths. Hence, according to the
fifteenth method realizing the aspect of the present invention, the
frequency range control units (51, 52, . . . , and 5N) can be switched to
different cut-off orders while the signal current ratio between the plural
transmission paths operated on the control voltages V1, V2, . . . , and VN
can be controlled without controlling the frequency range control circuits
501, 502, . . . , and 50N.
According to all the first to tenth methods, the frequency range control
units (51, 52, . . . , and 5N) is the frequency range control units (51,
52, . . . and 5N) with different cut-off orders and the frequency range
varying circuits are cascoded to control independently their frequency
ranges. Hence, according to the sixteenth method realizing the aspect of
the present invention, the frequency range control units (51, 52, . . . ,
and 5N) are switched to desired cut-off orders and the optimum signal path
is selected to control and vary the frequency range thereof.
According to the first to tenth method, the frequency range control units
(51, 52, . . . , and 5N) are frequency range control units (51, 52, . . .
, and 5N) having different cut-off orders. A voltage is added respectively
to the control voltages V1, V2, . . . , and VN to activate simultaneously
plural signal transmission paths, and the frequency range control units
are cascaded to control independently their frequency ranges. Hence
according to the seventeenth method realizing the aspect of the invention,
the frequency range control circuits (51, 52, . . . , and 5N) can be
switched to a desired cut-off order and can control the signal current
ratio between the plural transmission paths, without controlling the
frequency range control circuits 501, 502, . . . , and 50N. The signal
path can be switched selectively to the optimum signal one, thus
controlling and varying the frequency range thereof.
The first to seventeenth methods using bipolar transistors have been
described throughout the above explanations. The above-mentioned methods
can applicable to circuit systems using field effect transistors. In this
case, the base, emitter, and collector of the bipolar transistor
correspond to the gate, source, and drain of a field-effect transistor,
respectively.
(d) Explanation of the Embodiment of the Frequency Range Variable Filter
Circuit:
Next, the frequency-band variable filter circuit according to the
embodiment of the present invention will be described below with reference
to FIGS. 27 to 47.
In FIGS. 27 to 47, like symbols are attached to the same elements as those
shown in FIG. 26. Numerals 100 and 200 represent frequency band variable
filter circuits; 101, 101a, 101b, 102, 102a, 102b, 10N, 10Na, and 10Nb
represent emitter ground transistors; 201, 201a, 201b, 202, 202a, 202b,
20N, 20Na, 20Nb, 41 to 46, TR71, and TR72 represent transistors; 301,
301a, 301b, 302, 302a, 302b, 30N, 30Na, and 30Nb represent resistors (or
diodes); 401, 401a, 401a', 401b, 401b', 402, 402a, 402a', 402b, 402b',
40N, 40Na, 40Na', 40Nb, and 40Nb', represent cascade transistors; 501,
502, and 50N represent frequency range control circuits; 701 and 701a
represent constant current sources; 701b represents a variable current
source; 801, 801a, 801b, 901, 901a, 901b, 902, 902a, 902b, 90N, 90Na,
90Nb, and R71 to R73 represent resistors; R74 represents a variable
resistor; and VB1 and VB2 represent bias power sources.
FIG. 27 is a diagram showing the frequency band variable filter circuit
according to the first embodiment of the present invention. As shown in
FIG. 27, according to the present invention, the signal path switching
unit 21 shown in FIG. 26 is formed of the common-emitter transistor 101
having the emitter grounded via the resistor 901, and transistors 201,
202, . . . , and 2ON connected to the collector of the emitter ground
transistor 101.
The frequency range control units 51, 52, . . . , and 5N consists of the
frequency range control circuits 501, 502, . . . , and 50N connected
respectively to the collectors of the transistors 201, 202, . . . , and
20N, and the resistors 302, . . , and 30N with different impedances R2, .
. . , and RN connected respectively to the transistors 202, . . . , and
20N.
Moreover, the transistor 401 has the emitter connected to the collector of
the common-emitter transistor 101 and to the collectors of the
common-emitter transistors 202, . . . , and 20N respectively via the
resistors 302, . . . , and 30N.
In the circuit shown in FIG. 27, the collector signal current flowing
through the common-emitter transistor 101 varies its transmission path in
accordance with the control voltages V1, V2, . . . , and VN.
In other words, if VN>VM (M.noteq.N), the collector signal current flows
through the transistor 20N (N.noteq.1, 2, 3, . . . ).
If N=1, the frequency range f of the control unit is fCNTL(MIN)<FORG. Like
the conventional circuit, if N=1, the frequency fCNTL of the control unit
is expressed by the following formula:
fCNTL(MIN)=1/2.pi.(r.sub.e +RN).multidot.(Cd(IE)+CJE+CJC)
The minimum frequency range fCNTL(MIN) of the control unit is expressed by
the following formula:
fCNTL(MIN)=1/2.pi.(r.sub.e +RN).multidot.Cd(IE(MAX))
Hence, the minimum frequency range can be determined by the resistance
value RN of the resistor RN.
When the resistor RN is suitably selected and the control voltages V1, V2,
. . . , and VN select the transmission path, the frequency range can be
widely varied without degrading the maximum frequency range.
As described above, according to the first embodiment of the present
invention, the frequency band variable filter circuit can widely vary its
frequency range. Hence the frequency range of the amplifying circuit
arranged in the optical communications system can be set to the optimum
frequency range corresponding to the transmission speed of the received
signal. Thus, there is an advantage in that the amplifying circuit can
effectively amplify signals.
FIG. 28 is a diagram showing the frequency band variable filter circuit
according to the second embodiment of the present invention. The circuit
shown in FIG. 27 includes the common-emitter transistor 101 having the
emitter grounded via the resistor 901; the transistors 201, 202, . . . ,
and 20N; the resistors 302, . . . , and 30N; the frequency range control
circuits 501, 502, . . . , and 50N; and the cascade transistor 401.
However, the circuit shown in FIG. 28 has a differential pair
configuration which includes the current source 701 connected to the
commonemitter transistors 101a and 101b having the emitters connected to
the resistors 901a and 901b; the current source 701 connected to the
juncture between the resistors 910a and 901b; the transistors 201a, 201b,
. . . , 20Na, and 20Nb; the resistors 302a, 302b, . . . , 30Na, and 30Nb;
the frequency range control circuits 501, 502, . . . , and 50N; and the
cascade transistors 401a and 401b.
Like the first embodiment shown in FIG. 27, in the circuit shown in FIG.
28, the transistor differential pair 101a and 101b changes the
transmission path of the collector signal current in accordance with the
control voltage V1, V2, . . . , or VN.
In this case, if VN>VM (M.noteq.N), the collector signal current flows
through the transistor 20N (N=1, 2, 3, . . . ). If N.noteq.1, the minimum
frequency range fCNTL(MIN) of the control unit is 1/2.pi.(r.sub.e
+RN).multidot.Cd(IE(MAX)). Hence, since the minimum frequency range
fCNTL(MIN) of the control unit can be determined by selecting suitably the
resistance value RN of the resistor RN, the frequency range can be widely
varied by switching the transmission path by means of the control voltages
V1, V2, . . . , and VN, without degrading the maximum frequency range.
It is possible to vary the frequency range by constructing the filter
circuit in a differential configuration like the transistor differential
pair 101a and 101b.
According to the frequency band variable filter circuit of the second
embodiment of the present invention, like the first embodiment, the
frequency range of the amplifying circuit arranged in the optical
communications system, for example, can be matched with the optimum
frequency range corresponding to the received signal. Hence, there is an
advantage in that the amplifying circuit can effectively amplify the
signals.
FIG. 29 is a diagram showing the frequency band variable filter circuit
according to the third embodiment of the present invention. As shown in
FIG. 29, according to the present embodiment, the signal path switching
unit 21 shown in FIG. 26 is formed of the common-emitter transistor 101
grounded via the resistor 901, and the common-emitter transistors 102, . .
. , and 10N having the emitters grounded respectively via the resistors
902, . . . , and 90N.
FIG. 30 is a diagram showing the frequency band variable filter circuit
according to the fourth embodiment of the present invention. In FIG. 29,
the circuit system is formed of the common-emitter transistors 101, 102 .
. . , and ION, the resistors 302, . . . , and 30N, the frequency range
control circuits 501, 502, . . . , and 50N, and the cascade transistors
401. However, as shown in FIG. 30, the circuit system has a differential
pair configuration which includes the common-emitter transistors 101a,
101b, 102a, 102b, . . . , 10Na, and 10Nb having the emitters connected to
the constant current source 701 respectively via the resistors 901a, 901b,
902a, 902b, . . . , 90Na, and 90Nb; the resistors 302a, 302b, . . . ,
30Na, and 30Nb; the frequency range control circuits 501, 502 . . . , and
50N; and the cascade transistors 401a and 401b.
FIG. 31 is a diagram showing the frequency band variable filter circuit
according to the fifth embodiment of the present invention. As shown in
FIG. 31, the signal path switching units 21 formed of the common-emitter
transistors 101a, 102a, . . . , and 10Na, and 101b, 102b, . . . , and 10Nb
include differential transistor pairs 101a, 101b, 102a, 102b, . . . ,
10Na, 10Nb; the transistors 201 202, . . . , and 20N 202, . . . and 20N
having the collectors connected in common to the emitters of the
transistors 101a, 101b, 102a, 102b, . . . , 10Na, and 10Nb; and the
current source 701 connected in common to the emitters of the transistors
201, 202, . . . , and 20N.
In the fourth to sixth embodiments, the emitter ground transistors, or the
differential transistor pairs, can be switched by means of the control
voltages V1, V2, . . . , and VN. Since the control voltages V1, V2, . . .
, and VN are interposed to the input voltage Vin, the signal input
transistor 101 shown in FIG. 2 as well as the signal input transistors
101a and 101b shown in FIG. 28 can be omitted.
As described above, according to the frequency band variable filter circuit
of the fourth to sixth embodiments of the present invention, like the
second and third embodiments, the frequency range of the amplifying
circuit arranged in the optical communications system, for example, can be
matched with the optimum frequency range corresponding to the transmission
rate of the received signal. Hence, there is an advantage in that the
amplifying circuit can effectively amplify the signals and that the
differential transistor configuration can be easily realized as an
integrated filter circuit. Moreover, there is an advantage in that the
control voltage added to the input signal allows the simplified filter
circuit configuration, whereby an integrated filter circuit can be easily
realized.
The frequency control range can be widened as described with reference to
FIGS. 27 and 28.
FIG. 32 is a diagram showing the frequency band variable filter circuit
according to the sixth embodiment of the present invention. As shown in
FIG. 32, in the first embodiment (refer to FIG. 27), the filter circuit
includes the transistors 401', 402', . . . , and 40N' having the collector
connected in common to the emitter of the cascade transistor 401, the
emitter connected to the collector of the transistor 201 and to the
resistors 302, . . . , and 30N, and the base connected in common to the
bias power source VB2.
FIG. 34 is a diagram showing the frequency band variable filter circuit
according to the eighth embodiment of the present invention. As shown in
FIG. 34, in the third embodiment (refer to FIG. 29), the filter circuit
includes the cascade transistors 401', 402', . . . , and 40N' having the
collectors connected in common to the emitter of the transistor 401, the
emitters connected to collector of the emitter ground transistor 101 and
to the resistors 302, . . . , and 30N, and the bases connected in common
to the bias power source VB2.
In the first to fifth embodiments described with reference to FIGS. 27 to
31, when the resistor RN is relatively small at N=1, the parasitic
capacitance of the N (N.noteq.1)-th control circuit is apparently added in
parallel to the first range control unit, thus not being dumped by means
of the resistor RN, whereby the maximum frequency range is adversely
affected. However, the seventh to ninth embodiments relates to the circuit
configuration which can prevent the adverse effect.
That is, the degradation in the maximum frequency range can be avoided by
inserting the transistors 401', 402', . . . , and 40N' each which has the
emitter connected to the resistor RN, and the collector connected to the
emitter of the cascade transistor 401.
As described above, according to the frequency band variable filter circuit
of the seventh to ninth embodiments of the present invention, like the
above-mentioned embodiments, the frequency range of the amplifying circuit
arranged in the optical communications system, for example, can be matched
with the optimum frequency range corresponding to the transmission rate of
the received signal. Hence, there is an advantage in that the amplifying
circuit can effectively amplify the signals and the differential
transistor configuration can easily realize an integrated filter circuit.
Moreover, there is an advantage in that the differential transistor
configuration can easily realize as an integrated filter circuit. In
addition, since the cascaded transistors 401', 402', . . . , and 40N' can
advantageously avoid the degradation in the maximum frequency, the optimum
frequency range can be widened.
FIGS. 33, 34, and 36 are diagrams each showing the configuration of the
frequency band variable filter circuit relating to the seventh, ninth and
tenth embodiments of the present invention. The transistors 401a', 401b',
402a', 402b', . . . , 40Na', and 40Nb', each of which has the emitter
connected to the resistor RN and the collector connected to the emitters
of the transistors 401a', and 401b', are inserted to the circuit
configuration, shown in FIGS. 28, 30, and 31 according to the second,
fourth, and fifth embodiments. Thus, the degradation in the maximum
frequency range can be avoided.
For that reason, in the frequency band variable filter circuits according
to the seventh, ninth and tenth embodiments of the present invention
(shown in FIGS. 33, 35 and 36), the transistors 401', 402', . . . , and
40N' can avoid the degradation in the maximum range, in addition to the
effect described with the second, fourth, and fifth embodiments. Hence
there is an advantage in that the maximum frequency range can be broadly
obtained.
FIGS. 37(a), 37(b), and 37(c) are diagrams each explaining the operational
of the frequency band variable filter circuit according to the eleventh
embodiment of the according to the present invention. To describe the
operation of the above-mentioned circuit, the circuit of the second
embodiment, shown in FIG. 28, is shown below as an example. The circuit
configuration shown in FIG. 28, for example, has two signal transmission
paths. In the first to tenth embodiments, the control voltages V1 and V2
have amplitudes enough to cut off the transistors 201a and 202a, and 201b
and 202b necessarily. In the eleventh embodiment, for example, the range
control unit in the first signal transmission path is set to a broad
range, as shown in 37(a), while the range control unit in the second
signal transmission path is set to a narrow range, as shown in FIG. 37(b).
At the same time, a potential difference between the control voltages V1
and V2 has an amplitude at which the transistors 201a, 202a, 201b, and
202b are turned on.
As shown in FIG. 37(c), when only the first signal transmission path is in
on-state (or V1>>V2), the filter circuit exhibits the maximum frequency
range. When only the second signal transmission path is in on-state (or
V1<<V2), the filter circuit exhibits the minimum frequency range.
When the potential difference of the control voltage V1 turns on the
transistors 201a and 202a, and the potential difference of the control
voltage V2 turns on 201b and 202b, the filter circuit shows the frequency
characteristics with the two paths overlaid, as shown in FIG. 37(c).
Hence, the frequency range between the minimum range and the maximum range
can be controlled by setting arbitrarily the control voltages V1 and V2.
In the above explanation, the second embodiment has been explained.
However, the operation can be applied to the first embodiment and the
third to tenth embodiments.
FIG. 38 is a diagram showing the frequency band filter circuit according to
the twelfth embodiment of the present invention. The frequency band
variable filter circuits according to the first to eighth embodiments
(shown in FIGS. 27 to 34) are cascaded in a two stage. The frequency
characteristics of the frequency band variable filter circuit 100(A) is
shown in FIG. 39(a) while the frequency characteristics of the frequency
range variable filter circuit 200(B) is shown in FIG. 39(b). If the
frequency range fA of the filter circuit A and the frequency range fB of
the filter circuit B are controlled independently, when fA=fB, the
frequency characteristics of the cascaded circuit exhibit the composite
characteristics of the filter circuits A and B, as shown in FIG. 39(c).
The cut-off unit has a secondary order.
As shown in FIG. 39(c), if fA >>fB, the filter circuit B is dominant in the
frequency characteristic of the cascaded circuit and if fA<<fB, the filter
circuit A is dominant in the frequency characteristic of the cascaded
circuit. The cut-off unit has a primary order.
As described above, according to the twelfth embodiment of the present
invention, the frequency band variable filter circuit controls each
frequency range independently, thus setting the frequency range and the
cut-off order. Since the Nstage cascaded circuit can switch its cut-off
order from the first stage to the N-th stage, filter circuits with
different characteristics can be formed as a single integrated circuit.
This structure can suppress the circuit to the minimum scale and improve
largely the performance of the amplifying circuit.
In the present embodiment, the circuit has been described, which is formed
by cascading the circuits shown in the first to eighth embodiments. In
addition, the circuits according to the ninth to tenth embodiments can be
applied to the present embodiment.
FIG. 40 is a connection diagram showing the configuration where the
frequency band variable filter circuits 100 and 200 are cascaded in a
tandem form. This figure shows mainly the connection between the signal
output of the frequency band variable filter circuit 100 and the signal
input of the frequency band variable filter circuit 200.
Next, explanation will be made below as to three embodiments according to
the present invention. For simple explanation, two signal transmission
paths are shown.
FIG. 41 is a diagram showing the frequency band variable filter circuit
according to the second embodiment of the present invention (shown in FIG.
28). There are two signal transmission paths. The signal transmission path
is switched by a voltage added to the control voltages V1 and V2.
The frequency range control circuit along the first signal transmission
path consists of the transistors 401a and 401b, and the differential
transistor pair 501a and 501b.
The frequency range of the first signal transmission path is decided by the
emitter resistance r.sub.e of each of the cascade transistors 401a and
401b and the emitter diffusion capacitance Cd of each of the differential
transistor pair 501a and 501b. The frequency range can be controlled by
adjusting the current IE flowing through the differential transistor pair
501a and 501b.
The ranges fCNTL, fCNTL(MAX), and fCNTL(MIN) of the control unit can be
expressed by the following formulas:
fCNTL=1/2.pi.r.sub.e (Cd(IE)+CJE+CJC)
fCNTL(MAX)=1/2.pi.r.sub.e (CJE+CJC)
fCNTL(MIN)=1/2.pi.r.sub.e Cd(IE(MAX))
The frequency range control circuit along the second signal transmission
path consists of the transistors 401a and 401b; the diodes 302a and 302b
connected respectively and serially to the emitters of the transistors
401a and 401b; and the differential transistor pair 502a and 502b.
The frequency range of the second signal transmission path is decided by
the emitter resistance r.sub.e of each of the transistors 401a and 401b,
the output resistance r.sub.e of each of the diodes 302a and 302b
connected serially to the emitter resistors r.sub.e of the cascade
transistors 401a and 401b, respectively, and the emitter diffusion
capacitance Cd of each of the differential transistor pair 502a and 502b.
The frequency range can be controlled by adjusting the current IE flowing
through the differential transistor pair 502a and 502b.
The ranges f'CNTL, f'CNTL(MAX), and f'CNTL(MIN) of the control unit can be
expressed by the following formulas:
f'CNTL=1/2.pi.(r.sub.e +r.sub.d)(Cd(IE)+CJE+CJC)
f'CNTL(MAX)=1/2.pi.(r.sub.e +r.sub.d)(CJE+CJC)
f'CNTL(MIN)=1/2.pi.(r.sub.e +r.sub.d)Cd(IE(MAX))
Therefore, the entire frequency range controlled ranges from f'CNTL(MIN) to
f'CNTL(MAX).
FIG. 42 is a diagram showing the result obtained by simulating the circuit
shown in FIG. 41.
Referring to FIG. 42, the solid lines show the simulation result obtained
by controlling the current from the variable current source 701b when a
signal propagates along the first signal transmission path. The frequency
range can be varied by controlling the emitter current of each of the
transistors 501a, 501b, 502a, and 502b variable current source 701b, using
the variable current source 701b.
Dotted lines show the simulation result obtained by controlling the current
from the variable current source 701b when a signal propagates along the
second signal transmission path. The frequency range can be varied by
controlling the emitter current of each of the transistors 501a, 501b,
502a, and 502b, using the variable current source 701b.
Since the control voltages V1 and V2 switch the signal transmission path
switching and control the current from the variable current source 701,
the frequency range can be controlled to the extent shown with the solid
lines and the dotted lines.
FIG. 43 shows the fifth embodiment of the present invention shown in FIG.
31. There are two signal transmission paths. The signal transmission path
can be switched by means of a voltage added to the control voltages V1 and
V2. The frequency range control circuit operates, like that shown in FIG.
41. Hence the duplicate explanation will be omitted here.
FIG. 44 is a diagram showing the result obtained by simulating the circuit
shown in FIG. 43. This simulation result is similar to that shown in FIG.
42.
FIG. 45 shows the frequency band variable filter circuit according to the
tenth embodiment of the invention, related to that shown in FIG. 36. This
example shows two signal transmission paths. The signal transmission paths
can be switched by applying the control voltages V1 and V2.
The frequency range control circuit along the first transmission path
consists of the transistors 401a', and 401b', and the differential
transistor pair 501a and 501b. The frequency range control circuit along
the second transmission path consists of the transistors 402a', and 402b',
the resistors 302a and 302b connected serially to the emitters of the
transistors 402' and 402b', respectively, and the differential transistor
pair 502a and 502b.
The operation of the frequency range control circuit is similar to that
shown in FIG. 41. Hence the duplicate explanation will be omitted here.
FIG. 47(a) shows the configuration of the constant current source used for
the embodiments according to the present invention. FIG. 47(b) shows the
configuration of the variable current source used for the embodiments
according to the present invention. The current source with a current
Miller circuit, shown in FIG. 47(a), can produce the current lout at the
same value as that flowing through the transistor TR71.
The variable current source, shown in FIG. 47(b), can vary its output
current lout by adjusting the resistor R74.
The frequency band variable filter circuits including cascade transistors
or bipolar transistors ' acting as active elements have been described
throughout all the above-mentioned embodiments. However, in the
fundamental operation, similar circuits can be formed of field-effect
transistors.
Use of the field-effect transistors allows digital circuits to be
integrated very easily with the filter circuit on the same chip.
INDUSTRLAL APPLICABILITY
As described above, according to the present invention, the active
secondary low-pass filter circuit, the secondary high-pass filter circuit,
the secondary band-pass filter circuit, the resonance circuit, and the
frequency band variable filter circuit which can vary the cut-off
frequency and resonance frequency can be easily realized and integrated
without using discrete components such as coils and capacitors. Hence
these filter circuits are effective in improvement of its characteristics
and shrinkage in size. The filter circuits also are suitable to control
the frequency band of the amplifier that amplifies weakened signals
particularly used for the optical communications system to an optimum
value.
Top